Messages combine multiple buddies
If you select more than one contact in iChat or Apple’s new Messages beta for Mac (hold ⌘ or ⇧ when selecting multiple people in your Buddies list), the Buddies > Add Buddy command becomes Assign Buddies to One Address Card. This makes it easy to combine multiple usernames for the same person into a single card in Address Book.

As with any time when you’re tinkering with sensitive information you’d rather not lose, it’s a good idea to backup your Address Book contacts, be it with Time Machine through your Mac, iCloud or Google sync, or however else you roll.

via Jordan Running