If you’re an iChat user, you probably know how to add buddies to your buddy list—just select Buddies -> Add Buddy, or press Shift-Command-A. (You can also click the plus sign at the bottom of the window, and then select Add Buddy from the pop-up menu.) You can then enter their AIM address, or if they’re in your Address Book, you can type their first and/or last name to call up their record. When you see the person you’d like to add, click the Add button, and you’re done. This makes perfect sense, and works well—assuming you’re in iChat when you realize you want to add a buddy.
What usually happens to me, though, is that I’m usually in Address Book when I realize I want to add someone to my iChat buddy list—I’ll notice that a contact has an AIM (or more typically, mac.com) address, and decide to add them to iChat. Instead of toggling back to iChat, though, here’s a simpler solution: just drag and drop the contact’s name from Address Book directly into the iChat Buddy List, and that person will be added as a buddy.
Note that if you use the offline group (View -> Use Offline Group), you may not think this works—unless the contact you added happens to be online at the time you add their record to iChat, they’ll show up in the Offline group, not in the main list (or whatever group you dragged them into, if you also use View -> Use Groups option in iChat). But if you browse through the Offline group, you’ll see their entry amongst all your other logged off buddies.
This may be obvious to some, but when I found out about it, I’ll admit to never having tried to drag and drop an Address Book contact into the iChat Buddy List window.