authentication - Active directory form authentication and authorization

Question

I am trying to use active directory authentication for a web app using mvc5 c# form authentication. I have followed this example to make it work.

http://www.schiffhauer.com/mvc-5-and-active-directory-authentication/

However, using this method the AD group authorization doesnt work like this

 [Authorize(Roles = "Domain\\group_name")]

Is there an extension class that I can use to make it work like that ?