Giving Role to LDAP Authorization in ASP.NET MVC

I am using ASP.NET MVC and LDAP as authentication. It was successful. but I don't have any idea about how to give roles to the users. for example, I can login via LDAP with username "xxxx", and I want to give that "xxxx" role as "admin". then I can control the view with [Authorize(Roles="admin")]. How can I do that?