public override void AddUsersToRoles(string[] usernames, string[] roleNames)
{
   foreach (string roleItem in roleNames)
   {
      if (roleItem == "Administrator")
      {
         throw new ProviderException("Nie masz wystarczajcych uprawnie, aby doda" +
           " uytkownika do roli Administrator");
       }            
   }

   base.AddUsersToRoles(usernames, roleNames);
}
