Related link: http://www.leastprivilege.com/GettingAllGroupsForAWindowsAccountInNET20.aspx

Given the complexity of today’s Active Directory installations, the only safe way of getting all Windows groups a user is member of, is to inspect the token.

After you have acquired a token (e.g. though IIS authentication, LogonUser or Protocol Transition), wrap it in a WindowsIdentity and call:

List<string> getGroups(WindowsIdentity id)

{

  List<string> groups = new List<string>();

  IdentityReferenceCollection irc = id.Groups;

  foreach (IdentityReference ir in irc)

  {

    NTAccount acc = (NTAccount)ir.Translate(typeof(NTAccount));

    groups.Add(acc.Value);

  }

  return groups;

}