I think I've implemented something similar.
My solution, based on NerdDinner tutorial, is following.

When you sign the user in, add code like this:

var authTicket = new FormsAuthenticationTicket(
    1,                             // version
    userName,                      // user name
    DateTime.Now,                  // created
    DateTime.Now.AddMinutes(20),   // expires
    rememberMe,                    // persistent?
    "Moderator;Admin"                        // can be used to store roles
    );

string encryptedTicket = FormsAuthentication.Encrypt(authTicket);

var authCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encryptedTicket);
System.Web.HttpContext.Current.Response.Cookies.Add(authCookie);

Add following code to Global.asax.cs:

protected void Application_AuthenticateRequest(Object sender, EventArgs e)
{
    HttpCookie authCookie = Context.Request.Cookies[FormsAuthentication.FormsCookieName];
    if (authCookie == null || authCookie.Value == "")
        return;

    FormsAuthenticationTicket authTicket;
    try
    {
        authTicket = FormsAuthentication.Decrypt(authCookie.Value);
    }
    catch
    {
        return;
    }

    // retrieve roles from UserData
    string[] roles = authTicket.UserData.Split(';');

    if (Context.User != null)
        Context.User = new GenericPrincipal(Context.User.Identity, roles);
}

After you've done this, you can use [Authorize] attribute in your controller action code:

[Authorize(Roles="Admin")]
public ActionResult AdminIndex ()

Please let me know if you have further questions.

Build a custom AuthorizeAttribute that can use your enums rather than strings. When you need to authorise, convert the enums into strings by appending the enum type name + the enum value and use the IsInRole from there.

To add roles into an authorised user you need to attach to the HttpApplication AuthenticateRequest event something like the first code in http://www.eggheadcafe.com/articles/20020906.asp ( but invert the massively nested if statements into guard clauses!).

You can round-trip the users roles in the forms auth cookie or grab them from the database each time.

I did something like this:

• Use the Global.asax.cs to load the roles you want to compare in session,cache, or application state, or load them on the fly on the ValidateUser controller

Assign the [Authorize] attribute to your controllers, you want to require authorization for

 [Authorize(Roles = "Admin,Tech")]

or to allow access, for example the Login and ValidateUser controllers use the below attribute

 [AllowAnonymous] 

My Login Form

<form id="formLogin" name="formLogin" method="post" action="ValidateUser">
<table>
  <tr>
    <td>
       <label for="txtUserName">Username: (AD username) </label>
    </td>
    <td>
       <input id="txtUserName" name="txtUserName" role="textbox" type="text" />
    </td>
  </tr>
  <tr>
     <td>
         <label for="txtPassword">Password: </label>
     </td>
     <td>
         <input id="txtPassword" name="txtPassword" role="textbox" type="password" />
     </td>
  </tr>
  <tr>
      <td>
         <p>
           <input id="btnLogin" type="submit" value="LogIn" class="formbutton" />
        </p>
      </td>
  </tr>
</table>
       @Html.Raw("<span id='lblLoginError'>" + @errMessage + "</span>")
</form>

Login Controller and ValidateUser controller invoked from the Form post

Validate user is authentication via a WCF service that validates against the Windows AD Context local to the service, but you can change this to your own authentication mechanism

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Security;
using System.Security.Principal;
using MyMVCProject.Extensions;
namespace MyMVCProject.Controllers
{
public class SecurityController : Controller
{
    [AllowAnonymous]
    public ActionResult Login(string returnUrl)
    {
        Session["LoginReturnURL"] = returnUrl;
        Session["PageName"] = "Login";
        return View("Login");
    }
    [AllowAnonymous]
    public ActionResult ValidateUser()
    {
        Session["PageName"] = "Login";
        ViewResult retVal = null;
        string loginError = string.Empty;
        HttpContext.User = null;

        var adClient = HttpContext.Application.GetApplicationStateWCFServiceProxyBase.ServiceProxyBase<UserOperationsReference.IUserOperations>>("ADService").Channel;

        var username = Request.Form["txtUserName"];
        var password = Request.Form["txtPassword"];

        //check for ad domain name prefix
        if (username.Contains(@"\"))
          username = username.Split('\\')[1];

        //check for the existence of the account 
        var acctReq = new UserOperationsReference.DoesAccountExistRequest();
        acctReq.userName = username;
        //account existence result
        var accountExist = adClient.DoesAccountExist(acctReq);
        if (!accountExist.DoesAccountExistResult)
        {
            //no account; inform the user
            return View("Login", new object[] { "NO_ACCOUNT", accountExist.errorMessage });
        }
        //authenticate
        var authReq = new UserOperationsReference.AuthenticateRequest();
        authReq.userName = username;
        authReq.passWord = password;
        var authResponse = adClient.Authenticate(authReq);
        String verifiedRoles = string.Empty;
        //check to make sure the login was as success against the ad service endpoint
        if (authResponse.AuthenticateResult == UserOperationsReference.DirectoryServicesEnumsUserProperties.SUCCESS)
        {
            Dictionary<string, string[]> siteRoles = null;

            //get the role types and roles
            if (HttpContext.Application["UISiteRoles"] != null)
                siteRoles = HttpContext.Application.GetApplicationState<Dictionary<string, string[]>>("UISiteRoles");

            string groupResponseError = string.Empty;
            if (siteRoles != null && siteRoles.Count > 0)
            {
                //get the user roles from the AD service
                var groupsReq = new UserOperationsReference.GetUsersGroupsRequest();
                groupsReq.userName = username;
                //execute the service method for getting the roles/groups
                var groupsResponse = adClient.GetUsersGroups(groupsReq);
                //retrieve the results
                if (groupsResponse != null)
                {
                    groupResponseError = groupsResponse.errorMessage;
                    var adRoles = groupsResponse.GetUsersGroupsResult;

                    if (adRoles != null)
                    {
                        //loop through the roles returned from the server
                        foreach (var adRole in adRoles)
                        {
                            //look for an admin role first
                            foreach (var roleName in siteRoles.Keys)
                            {
                                var roles = siteRoles[roleName].ToList();
                                foreach (var role in roles)
                                {
                                    if (adRole.Equals(role, StringComparison.InvariantCultureIgnoreCase))
                                    {
                                        //we found a role, stop looking
                                        verifiedRoles += roleName + ";";
                                        break;
                                    }
                                }
                            }
                        }
                    }
                }
            }
            if (String.IsNullOrEmpty(verifiedRoles))
            {
                //no valid role we need to inform the user
                return View("Login", new object[] { "NO_ACCESS_ROLE", groupResponseError });
            }

            if (verifiedRoles.EndsWith(";"))
                verifiedRoles = verifiedRoles.Remove(verifiedRoles.Length - 1, 1);

            //all is authenticated not build the auth ticket
            var authTicket = new FormsAuthenticationTicket(
            1,                             // version
            username,                      // user name
            DateTime.Now,                  // created
            DateTime.Now.AddMinutes(20),  // expires
            true,                    // persistent?
           verifiedRoles   // can be used to store roles
            );

            //encrypt the ticket before adding it to the http response
            string encryptedTicket = FormsAuthentication.Encrypt(authTicket);

            var authCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encryptedTicket);
            Response.Cookies.Add(authCookie);

            Session["UserRoles"] = verifiedRoles.Split(';');

            //redirect to calling page
            Response.Redirect(Session["LoginReturnURL"].ToString());
        }
        else
        {
            retVal = View("Login", new object[] { authResponse.AuthenticateResult.ToString(), authResponse.errorMessage });
        }

        return retVal;
    }
}

}

User is authenticated now create the new Identity

protected void FormsAuthentication_OnAuthenticate(Object sender,     FormsAuthenticationEventArgs e)
    {
        if (FormsAuthentication.CookiesSupported == true)
        {
            HttpCookie authCookie = Context.Request.Cookies[FormsAuthentication.FormsCookieName];
            if (authCookie == null || authCookie.Value == "")
                return;

            FormsAuthenticationTicket authTicket = null;
            try
            {
                authTicket = FormsAuthentication.Decrypt(authCookie.Value);
            }
            catch
            {
                return;
            }

            // retrieve roles from UserData
            if (authTicket.UserData == null)
                return;

            //get username from ticket
            string username = authTicket.Name;

            Context.User = new GenericPrincipal(
                      new System.Security.Principal.GenericIdentity(username, "MyCustomAuthTypeName"), authTicket.UserData.Split(';'));
        }
    }

On my site at the the top of my _Layout.cshtml I have something like this

 {
  bool authedUser = false;
  if (User != null && User.Identity.AuthenticationType == "MyCustomAuthTypeName" && User.Identity.IsAuthenticated)
   {
      authedUser = true;
   }
 }

Then in the body

        @{
         if (authedUser)
          {
            <span id="loggedIn_userName">
                <label>User Logged In: </label>@User.Identity.Name.ToUpper()
            </span>
          }
          else
          {
            <span id="loggedIn_userName_none">

                <label>No User Logged In</label>
            </span>
          }
        }

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Security;
using SISWEBBSI.Models.Model;
using SISWEBBSI.Models.Model.Entities;
using SISWEBBSI.Models.ViewModel;

namespace SISWEBBSI.Controllers.ActionFilter
{
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, Inherited = true, AllowMultiple = false)]
    public sealed class RequerAutorizacao : ActionFilterAttribute
    {
        public Grupo.Papeis[] Papeis = {} ;
        public string ViewName { get; set; }
        public ViewDataDictionary ViewDataDictionary { get; set; }
        public AcessoNegadoViewModel AcessoNegadoViewModel { get; set; }

        public override void OnActionExecuting(ActionExecutingContext FilterContext)
        {
            if (!FilterContext.HttpContext.User.Identity.IsAuthenticated)
            {
                string UrlSucesso = FilterContext.HttpContext.Request.Url.AbsolutePath;
                string UrlRedirecionar = string.Format("?ReturnUrl={0}", UrlSucesso);
                string UrlLogin = FormsAuthentication.LoginUrl + UrlRedirecionar;
                FilterContext.HttpContext.Response.Redirect(UrlLogin, true);
            }
            else
            {
                if (Papeis.Length > 0)
                {
                    //Papel ADMINISTRADOR sempre terá acesso quando alguma restrição de papeis for colocada.
                    int NovoTamanho = Papeis.Count() + 1;
                    Array.Resize(ref Papeis, NovoTamanho);
                    Papeis[NovoTamanho - 1] = Grupo.Papeis.ADMINISTRADOR;
                    UsuarioModel Model = new UsuarioModel();
                    if (!Model.UsuarioExecutaPapel(FilterContext.HttpContext.User.Identity.Name, Papeis))
                    {
                        ViewName = "AcessoNegado";
                        String Mensagem = "Você não possui privilégios suficientes para essa operação. Você deve estar nos grupos que possuem";
                        if(Papeis.Length == 1)
                        {
                            Mensagem = Mensagem + " o papel: <BR/>";
                        }
                        else if (Papeis.Length > 1)
                        {
                            Mensagem = Mensagem + " os papéis: <BR/>";
                        }

                        foreach (var papel in Papeis)
                        {
                            Mensagem = Mensagem + papel.ToString() + "<br/>";
                        }
                        AcessoNegadoViewModel = new AcessoNegadoViewModel();
                        AcessoNegadoViewModel.Mensagem = Mensagem;
                        ViewDataDictionary = new ViewDataDictionary(AcessoNegadoViewModel);
                        FilterContext.Result = new ViewResult { ViewName = ViewName, ViewData = ViewDataDictionary };
                        return;
                    }
                }
            }
        }
    }
}

Add your users to the table "users in roles". Use the stored procedure "addusertorole" (something like that) in your code to add to various roles. You can create the roles very simply in the "roles" table.

Your tables to use: User, UsersInRole, Roles

Use the built in Stored Procs to manipulate those tables. Then all you have to do is add the attribute.

For example you can have an "Admin" attribute on a view that selects a user and adds them to a role. You can use the stored proc to add that user to the role.