By default, employees are listed alphabetically according to their first name (First Last).
If you would like to arrange employees according to their last name, you can easily update this.
You'll find it under your preferences page (Dashboard > Settings > Preferences).
Simply choose your preferred format, then save, and all parts of Sling will be updated to reflect your choice.
Admins and managers, keep in mind that this is a user preference, so it will not affect your employees' views.