How to Add Buddies and Groups to Your Mac Messages App Buddy List

Although you don’t have to use the Friends List for all your friends and contacts on your Mac, it can come in handy. To summon your friends list, choose Window → Buddies.
In the friends list, you can add and organize contacts:
Add a new buddy to the list: Click + in the lower left corner of the Friends window and then choose Add Friend from the resulting list. In the window that appears, type in your friend’s AIM or Yahoo! Or Gmail or Mac.com, as well as their first and last real name in the custom fields. You can also add the new friend to a group.
Alternatively, choose an entry from your Contacts app by clicking the down arrow in the lower right corner of the Add Friends window. The person’s name appears instantly in your friends list.
Add a group (maybe co-workers, soccer team, etc.): After clicking + in the upper-right corner of the field where you want to direct your message, which calls up your contacts, click on the name of the group. You must have already set up a group within Contacts on your Mac (or on another device via iCloud) for the group name to appear here.