public class PrincipalService
{
     const int MaximumDepth = 100;

     public static IEnumerable<string> GetGroups(string identity)
     {
         var ctx = new PrincipalContext(ContextType.Domain);
         var user = UserPrincipal.FindByIdentity(ctx, identity);
         return GetGroups(user, 0).Distinct();
     }

     private static IEnumerable<string> GetGroups(Principal principal, int currentDepth)
     {
         foreach (var group in principal.GetGroups())
         {
             yield return group.Name;
             if (currentDepth < MaximumDepth)
             {
                 foreach (string sub in GetGroups(group, currentDepth + 1))
                 {
                     yield return sub;
                 }
             }
         }
     }
} 

groupPrincipal.GetMembers(true)