C# access user roles

Question

In a C# 2008 desktop application, I need change the application since all the users are moving from one domain to new domain. The current application does not let the user access information under the new domain name.
To accomplish this goal, I would like to see what roles have been setup for those users starting with myself.
To accomplish this goal, I would like to know how to access those roles.

I do know that when I look at: Thread.CurrentPrincipal.Identity.Name, I can access the person's name. I also know that if I use System.Security.Principal.WindowsIdentity.GetCurrent(), I get the same information.

However can you tell me in code how to access the subcategories in the following:
1. Thread.CurrentPrincipal.Identity
2. System.Security.Principal.WindowsIdentity

Can you tell me in tell or show me in code how to accomplish this goal?