HI,
It would be helpful to know if these are iMessages you are Sending or IMs via AIM or Jabber ?
iMessages needs your Apple ID to be "registered" in the Messages set up process.
It may be linked to your iPhone or iPad (or Touch iPod) if you use the same Apple ID there.
Messages that ar then sent and received are then displayed across all devices using that ID.
You can also elect to Send from the iPhone Number on your Mac if you are using iOS 6 an Mac OS X 10.8.2 (your specs suggest not)
Messages can also be set up to Log in to AIM or Jabber (GoogleTalk and Facebook IDs are valid Jabber IDs)
Lets presume you are chatting using iMessage.
You would click the icon top left (New Message)
You would then see the blue + icon top right
Using that you can select "Contacts" and an Individual contact.
You can add a Second name in the "To" spot. (repeat for many contacts in the same chat).
(you can of course type the contacts name in the "To" spot and then chose their iMessages details to use.)
So the names must be linked to their iMessages details with this method.
The typing you do should appear on the right hand side (If using the Balloons Style) when you press the Enter/Return key.
IF you are using a AIM Name yourself it tends to be better to show the Buddy Lists (See Window Menu > Buddies.
Double clicking a Buddy will get them listed in the Messages Window automatically listing your AIM account as the sending name. (and their AIM name as the "receiving" one).
Again extra Buddies can be added at the same time (Various Modifier keys and a click can highlight buddies next to each other in the list or separated ones)
Or you can go to the File Menu > Go to Chat room and enter of create a room.
There is little difference with Jabber IDs except in some cases you may find the Server involved may need setting up to allow you to group chat. (Tends to need a third party app)
9:10 PM Tuesday; November 13, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
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"Limit the Logs to the Bits above Binary Images." No, Seriously