On the subsequent screen, drag down anywhere below the “Details” header to reveal the “Group Name” field.
Type anything you’d like in there, and that’s all there is to it!
Under Yosemite, it’s just as simple. Open the Messages program, click on a group conversation, and then again click on the “Details” option (it’s in the upper-right corner):
As you see above, “Untitled Group Chat” will appear at the top of the small window that drops down, and that’s where you’ll type in the new chat name. Neat!
Uh oh, here come all the caveats. Big ones. Be aware that the other participants can see any titles you add, so please don’t name the conversation with your bosses “Chatting with the Idiot Mouth-Breathers,” OK?
Keep in mind, too, that an in-chat notification will appear for the change you made…
…and new notifications the members get for the conversation will include the group name you’ve applied.
So, yeah. This is a fun feature, and I dig it, but because it shares that info with all of the participants, I wouldn’t use this for anyone other than my closest friends. They’re already accustomed to my weirdness, so I don’t have to worry that they’ll stop talking to me. I hope.