SIEGE's IRC channel is located on the Freenode network, in #siege.
There are multiple ways to connect to the channel:
With your own IRC client
Connect to irc://irc.freenode.net/#siege (* NOTE below)
With PJIRC, a java IRC client
The client is located on this website: http://irc.libsiege.org/pjirc
With Freenode's AJAX-based client
You can find it here: http://webchat.freenode.net/?channels=#siege
* NOTE: The Mibbit IRC client is blocked by Freenode due to a large amount of spammers using Mibbit as a proxy.