We hang out on #ALUG on the http://www.oftc.net/ OFTC network; this can be found by pointing your IRC client to irc.oftc.net (or alternatively irc.alug.org.uk, which points to OFTC).
Connection to oftc is covered at http://www.oftc.net/oftc/OFTC
IRC stands for Internet relay chat. It is a kind of online chat room system where people can talk even when they are in different locations. It is a bit like a telephone conference line but with a text interface.
Technically IRC meetings are held every Monday evening at around 8pm onwards, but in general you can turn up at any point and there will be a bunch of people around - there are a handful of members who are regularly on from work as the channel can be both a useful resource and way to let off steam.
Some of the common people on channel and their real names are listed on IRCNames
Popular irc clients include Irssi (apparently the channel's text mode client of choice) and X-Chat. Here are a few links to client to get you started.
EPIC - Enhanced Programmable ircii Client http://www.epicsol.org/
http://freshmeat.net/ (lots of them)
Mac OS X
Ircle http://www.ircle.com/ (shareware)
Should also run the Unix clients above.