Return a list of (blog post) node authors, as user objects sorted by last name.

gistfile1.txt

function cmp_user_last_name($user1, $user2) {
  return strcmp($user1->field_user_last_name, $user2->field_user_last_name);
}

function sorted_authors($nid) {
  // get the author uids from the content node
  $node = node_load($nid);
  $authors = reset($node->field_authors);

  global $user;
  $authors = array_map(user_load, $authors); 
  usort($authors, cmp_user_last_name);

  return $authors;
}