For an interactive real-time group discussion I strongly suggest Skype as it runs on Windows, MacOS X and Linux and is completely free. It also has some useful functions, especially with Unyte (also free) for screen sharing. You can really help someone out when you can see their screen as you talk with them. Unyte works as a Java applet in the web browser so that is also cross-platform.
I administer a Jabber server, if you like I could host a Jabber conference room for Roundcube developers. A friend of mine has written a jabber to IRC gateway, which means participants could use either Jabber or IRC and talk to each other.
Just throwing that out there,
Doug