I figured out how to do this.
Edit the following file from the add-on:
library >> XenTrCom >> WhoHasVisited >> Model >> XWhoHasVisited.php
Look for this code (it's on line 40 of my file)...
Code:
$WHVArray = $db->fetchAll("SELECT user_id FROM xf_user as user WHERE last_activity BETWEEN '$datemin' AND '$date' $bannedExcluded $userPrivacy ORDER BY last_activity DESC");
At the end of the line, change......
ORDER BY last_activity DESC");
to
ORDER BY username");