Попробуйте это в .NET 3.5 или 4:
PrincipalContext infPC = new PrincipalContext(ContextType.Domain, "domain", "login", "password");
UserPrincipal infUP = new UserPrincipal(infPC);
PrincipalSearcher infPS = new PrincipalSearcher();
UserPrincipal foundUP;
GroupPrincipal infGP = new GroupPrincipal(infPC);
GroupPrincipal foundGP;
string CurrentUser = WindowsIdentity.GetCurrent().Name;
infUP.SamAccountName = CurrentUser;
infPS.QueryFilter = infUP;
foundUP = infPS.FindOne();
infGP.Name = "CustomGroup";
infPS.QueryFilter = infGP;
foundGP = infPS.FindOne();
bool ismember = foundUP.IsMemberOf(foundGP);