First of all, note that I never got groupchat to work until I checked out and built the CVS source. (The "confencing.tgz" file on download.jabber.org probably won't work: do a real cvs checkout.)
Then, the following bits in my jabber.xml file. Obviously, adjust server names and paths to suit your taste.
First, inside the <jabber></jabber> object:
<service id="conference.mspt.com"> <load><groupchat>/local/vol/jabber/lib/groupchat.so</groupchat></load> <config xmlns="jabberd:gc:config"> <history>20</history> </config> </service> <log id="gc_log"> <file>/local/vol/jabber/log/grouchat.log</file></log>
Then, inside the <agents></agents> section:
<agent jid="conference.mspt.com"> <name>Conference Server</name> <description>You can create and participate in private group conferences.</description> <service>conference.mspt.com</service> <groupchat/> <register/> </agent>
And that, believe it or not, was it.