This is a tough one. Sometimes it works and sometimes it does not. I have found that if you install the new Messages beta, you have a better shot at it working.
Here are some other variations we have noticed:
1.) Sometimes it takes two tries. As in, close out the group text chat that did not work and try again.
2.) Pressing the "A" key to start the chat after you have selected the party, sometimes and sometimes does not work. So, when it does not work, then double-click someone's name in the group to initiate the chat.
Point it, it is not working smoothly, if at all a lot of the time. When it doesn't work, you don't know if folks are just not responding or if they did not get the chat.
Also, we have tried iChat group text chat with vpn and no vpn outside of the local network and it does not work. For anyone outside, you are out of luck. We are running the latest as of this writing, 10.7.4 and so is the Lion server. Note that all other network services work fine for us outside of the network.