The instructions for joining the Great Hall Chat can be found HERE. HOWEVER, the old Great Hall Chat is broken, so anywhere that it says "irc.alexisnet.net" needs to be replaced with irc.esper.net.
If you are familiar with IRC, the jist of it is:
Server: irc.esper.net
Channel: #HiHGreatHall.
Come on in and join the party!! :D And congratulations Gryffindor!