Google Talk uses Jabber which has support for group chats. Hosting your own jabber server isn't a huge deal. You could use IRC for group chats and have multiple rooms there as well. Again, it's not a huge deal to setup your own IRC server.
It'd be nice if there were some free/open-source software services listed there, but oh well.
Just because you can set something up yourself doesn't mean you should. You'll take on the burden of maintenance, support, and teaching people how to use the system. We could all be running git on our own servers instead of using GitHub, our own mailservers with Postfix and Horde instead of using Gmail, and using rsync instead of Dropbox. But for most of us it's worth it to pay for a managed service with an improved user experience. This is especially true in a startup where time is the most valuable resource you have.
We tried to run our own IRC server. It's not exactly that easy. For example, you also need to deal with authentication, as well as set up an IRC proxy just to get backlogs on reconnect. Never mind the fact that IRC doesn't have important features like inline images or nice iPhone apps with notifications, etc.
We tried IRC, GTalk/Jabber, FlowDock and Campfire, but all of them were kinda "meh". HipChat is actually totally rad. Try it out: http://hipchat.com/r/2zqso
I was going to call you out for recommending HipChat and then posting a referral link, but then I saw that the referral merely gets you a custom icon that they draw for you. Pretty interesting incentive.
For anyone who is wondering, for Android phones (not sure about iOS) there is an app called andchat which handles IRC extremely well, definitely worth checking out.
It'd be nice if there were some free/open-source software services listed there, but oh well.