The interface becomes unusable with dropdown and more than a line of users

Ok. I finally found the solution.

Version: 6.x-2.1-beta8
File: notifications_team.module
Line: 775.
Fixing: Remove $prefix variable from $matches at the autocomplete function.
Result: The drop down just show one user, so you can add as many users as you like.

function notifications_team_autocomplete($oggroup = 0, $notify = '') {
  // The user enters a comma-separated list of destinations. We only autocomplete the last tag.
  $array = split(',', $notify);
  foreach ($array as $key => $entry) {
    $array[$key] = trim($entry);
  }

  $string = trim(array_pop($array));
  $matches = array();
  if ($string) {
    $prefix = count($array) ? implode(', ', $array) .', ' : '';
    foreach (notifications_team_get_users($string, 10, $oggroup) as $user) {
      $display = isset($user->plaintext) ? $user->plaintext : check_plain($user->name);
      $matches[$prefix . $user->name] = $display; // $display . $prefix; <---------- Remove $prefix variable.
    }
  }
  drupal_json($matches);
}

Sorry, but now I can't submit a patch.

Here it is the patch.

Please, take in consideration for next release.

Thanks a lot for the great module,
m.