Hi,
GoogleTalk is supposed to allow group chats.
These tend to be called "Conferences" on most Jabber servers.
Most Jabber servers have a section of additional itme your can "register" for and Login to.
However to do this takes a third party app like Psi.
This can then do "Service Discovery" on the server in question and show what features you can sign up for.
These can include "Transports" that allow you to add Buddies from different services and Conference calls.
Having said that if you do Service Discovery on GoogleTalk you find nothing
Many apps also presume the server name is the last part of your user ID and for Google with it's @gmail.com and @googlemail.com this does not work properly in some of these apps.
This page has Instructions for how to set up a Transport to MSN using Psi and you can see the Service Discovery panel.
Essentially a room has to be given a Name and this is coupled with the Jabber server name it is on so that it looks something like this:-
ralph@conference.jabber.org.uk
The room "ralph" is on the conference server at the jabber.org.uk server.
Substituting ralph@conference.talk.google.com does not seem to work as a room name.
It may pay to start from a Web Browser login and see if a room name becomes apparent that way.
Edit
The last two "links" I have tried to remove the Links but they keep get added back

9:53 PM Friday; April 26, 2013
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
Message was edited by: Ralph Johns (UK)
Message was edited by: Ralph Johns (UK)
Message was edited by: Ralph Johns (UK)