User list has already been sorted by the SQL request in User::getSqlSearchResult() function user.class.php
Extract of code from user.class.php:

`if ($_SESSION["glpinames_format"] == self::FIRSTNAME_BEFORE) {
            $query.=" ORDER BY `glpi_users`.`firstname`,
                               `glpi_users`.`realname`,
                               `glpi_users`.`name` ";
         } else {
            $query.=" ORDER BY `glpi_users`.`realname`,
                               `glpi_users`.`firstname`,
                               `glpi_users`.`name` ";
         }`