That will be the Bonjour Buddy List.
The Buddies are added to this one automatically.
It picks up the other people using Macs and with iChat ON (or Messages) with the Bonjour account enabled.
You see their Name as it is in their Address Book > My Card.
They see your Name as per your My Card in the Address Book (Contacts app on Mountain Lion)
You will need to set up iChat with either an AIM name or a Jabber based one.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
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