I must admit I don't know how I would even go about that.
Logic tells me it should be something to do with an Contacts/Address Book Distribution List and having all the Apple IDs listed.
Although I can use the Groups I have in the Contacts app as a Distribution list it seems I cannot do the same in Messages.
I can see that you could add all the names manually the first time but I see no real way to "Save" that in any way for iMessages.
The iMessages side does not use the Go to Chatroom option (File Menu) when doing Multi chats.
I do know that 2 and 3 way iMessages are possible as I have tried that and even sent in line Pics that way.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously