How to Add Buddies and Groups to Your Mac Messages App Buddy List
Though you don’t have to use the buddy list for all your friends and contacts on your Mac, it can be helpful. To summon your list of Buddies, choose Window→Buddies.
In your Buddy List, you can add and organize your contacts:
Add a new buddy to the list: Click the + at the lower-left corner of the Buddies window and then choose Add Buddy from the resulting menu. In the window that appears, type your buddy’s AIM, Yahoo!, Gmail, or Mac.com account, plus his or her real first and last names in the designated fields. You can also add the new buddy to a group.
Alternatively, choose an entry from your Contacts app by clicking the downward-pointing arrow in the lower-right corner of the Add Buddy window. The person’s name turns up instantly in your buddy list.
Add a group(perhaps your coworkers, soccer team, and so on): After clicking the + at the upper-right corner of the field in which you want to address your message, which summons your Contacts, click Group Name. You have to have already set up a group within Contacts on your Mac (or on another device via iCloud) for the group name to appear here.