Winforms Authentication

I am trying to set up authentication for my winforms app (framework version 3.5), and I'm having a little trouble. I set the default provider to ClientAuthenticationMembershipProvider, and I am able to validate the user with Membership.ValidateUser. What I can't figure out is how to properly retrieve the current users information (id, username etc). Thread.CurrentPrincipal.Identity seems to give me the username and will tell me if they are authenticated, but it doesn't include the users id (a guid). What am I missing?