John Kolbert wrote a function to to get a list (array) of users by their role name. For example, he shows how to get a list of editors. I would only change the function slightly to accommodate my own programming style (and because I can't leave well enough alone).
  1. function get_users_with_role ($role) {
  3.     // gets all users with specified role
  4.     $wp_user_search = new WP_User_Search( '', '', $role);
  6.     return $wp_user_search->get_results();
  8. }

As John notes, remember this is for use in the admin panels. Thanks, John.