The default for AIM for Mac, your buddies in the Buddy List are displayed in groups, with the last one being the Offline Buddy group. You will initially see all of your offline buddies in the Offline group, and when they come online, they will be moved to their pre-assigned online group.
You can hide the Offline group by clicking on the View menu and selecting Offline Buddies > Do Not Show. Then, your Offline group will disappear.
You can also show your offline buddies within each of the groups by clicking on the View menu and selecting Offline Buddies > Show in Groups. You will still not have an Offline group and all of your offline buddies will be displayed.
Experiment with these offline settings to find the display the suits you the best. To find more configuration options for the Buddy List, look in the application preferences.