public override void AddUsersToRoles(string[] usernames, string[] roleNames)
{
   foreach (string roleItem in roleNames)
   {
      if (roleItem == "Administrator")
      {
         throw new ProviderException("You are not authorized to add any users" +
           " to the Administrator role");
       }            
   }

   base.AddUsersToRoles(usernames, roleNames);
}
