Q:

How would I mimic User.IsInRole()

I have a website thats build with VS 2012 Internet Application ( Simple membership) EF Code First

Updates 
I would like to know how to extend HttpContext.User.IsInRole(role) 's functionality for a custom table -> User.IsInClient(client).

A:

Here is the way I'd suggest to solve your issue:
Create your own interface which implements System.Security.Principal, where you could place any methods you need:
public interface ICustomPrincipal : IPrincipal
{
    bool IsInClient(string client);
}

Implement this interface:
public class CustomPrincipal : ICustomPrincipal
{
    private readonly IPrincipal _principal;

    public CustomPrincipal(IPrincipal principal) { _principal = principal; }

    public IIdentity Identity { get { return _principal.Identity; } }
    public bool IsInRole(string role) { return _principal.IsInRole(role); }

    public bool IsInClient(string client)
    {
        return _principal.Identity.IsAuthenticated 
               && GetClientsForUser(_principal.Identity.Name).Contains(client);
    }

    private IEnumerable<string> GetClientsForUser(string username)
    {
        using (var db = new YourContext())
        {
            var user = db.Users.SingleOrDefault(x => x.Name == username);
            return user != null 
                        ? user.Clients.Select(x => x.Name).ToArray() 
                        : new string[0];
        }
    }
}

In the Global.asax.cs assign your custom principal to the request user context (and optionally to the executing thread if you plan to use it later). I suggest to use Application_PostAuthenticateRequest event not Application_AuthenticateRequest for this assignment, otherwise your principal will be overridden (at least by ASP.NET MVC 4):
protected void Application_PostAuthenticateRequest(Object sender, EventArgs e)
{
    Context.User = Thread.CurrentPrincipal = new CustomPrincipal(User);

    /* 
     * BTW: Here you could deserialize information you've stored earlier in the 
     * cookie of authenticated user. It would be helpful if you'd like to avoid 
     * redundant database queries, for some user-constant information, like roles 
     * or (in your case) user related clients. Just sample code:
     *  
     * var authCookie = Request.Cookies[FormsAuthentication.FormsCookieName];
     * var authTicket = FormsAuthentication.Decrypt(authCookie.Value);
     * var cookieData = serializer.Deserialize<CookieData>(authCookie.UserData);
     *
     * Next, pass some deserialized data to your principal:
     *
     * Context.User = new CustomPrincipal(User, cookieData.clients);
     *  
     * Obviously such data have to be available in the cookie. It should be stored
     * there after you've successfully authenticated, e.g. in your logon action:
     *
     * if (Membership.ValidateUser(user, password))
     * {
     *     var cookieData = new CookieData{...};         
     *     var userData = serializer.Serialize(cookieData);
     *
     *     var authTicket = new FormsAuthenticationTicket(
     *         1,
     *         email,
     *         DateTime.Now,
     *         DateTime.Now.AddMinutes(15),
     *         false,
     *         userData);
     *
     *     var authTicket = FormsAuthentication.Encrypt(authTicket);
     *     var authCookie = new HttpCookie(FormsAuthentication.FormsCookieName, 
                                           authTicket);
     *     Response.Cookies.Add(authCookie);
     *     return RedirectToAction("Index", "Home");
     * }
     */         
}

Next, to be able to use the property User from HttpContext in the controller without casting it to ICustomPrincipal each time, define base controller where you override the default User property:
public class BaseController : Controller
{
    protected virtual new ICustomPrincipal User
    {
        get { return (ICustomPrincipal)base.User; }
    }
}

Now, let other controllers inherit from it:
public class HomeController : BaseController
{
    public ActionResult Index()
    {
        var x = User.IsInClient(name); 

If you use Razor View Engine, and you'd like to be able to use your method in the very similar way on the views:
@User.IsInClient(name)

you need to redefine WebViewPage type:
public abstract class BaseViewPage : WebViewPage
{
    public virtual new ICustomPrincipal User
    {
        get { return (ICustomPrincipal)base.User; }
    }
}

public abstract class BaseViewPage<TModel> : WebViewPage<TModel>
{
    public virtual new ICustomPrincipal User
    {
        get { return (ICustomPrincipal)base.User; }
    }
}

and tell Razor to reflect you changes, by modifying appropriate section of the Views\Web.config file:
<system.web.webPages.razor>
    ...
    <pages pageBaseType="YourNamespace.BaseViewPage">

