I think currently usernames in the sidebar are ordered by last activity, yes? Is it possible to order them chronologically (A first, Z last) by username?
You will have to edit some code in Model/Session.php
In function getSessionActivityQuickList(), line 55
change
'order' => 'view_date'
to
'order' => 'username'
Well, all that I can tell is that I applied the change in Syndol's post and it's working in the side bar only. According to your thumbnail it should also affect the recent activity and online page, but for me it didn't.