public static List<string> GetUserGroups( string userName)
        {
            List<string> groups = new List<string>();
            try
            {
                using (var entry = GetRootDirectoryEntry())
                using (var searcher = new DirectorySearcher(entry, string.Format("(sAMAccountName={0})", userName)))
                {
                    var result = searcher.FindOne();
                    if (result != null)
                    {
                        using (var user = new DirectoryEntry(result.Path))
                        {
                            foreach (object ob in user.Invoke("Groups") as IEnumerable)
                            {
                                using (var group = new DirectoryEntry(ob))
                                {
                                    groups.Add(group.Name);
                                }
                            }
                        }
                    }
                }
            }
            catch
            {

            }
            groups.Sort();
            return groups;

        }

  public static DirectoryEntry GetRootDirectoryEntry()
        {
            using (DirectoryEntry defaultServer = new DirectoryEntry(string.Format("LDAP://rootDSE")))
            {
                string strLdapServer = (string)defaultServer.Properties["defaultNamingContext"].Value;
                return new DirectoryEntry("LDAP://" + MembershipSettings.DomainCompany);
            }
        }