Google Talk Gadget added support group chat. Just click on the "Group Chat" button when you are in a conversation, and you can invite other people to join your discussion. Unfortunately, this feature is not available in any other client, so if you invite someone who uses Google Talk for Windows, Gmail Chat or other Jabber client, he'll get a link to the web version of Google Talk.
The Windows client seems to be neglected this year, but I bet we'll see a major new release in the coming weeks that will add all the new features from the web version, phone calls and more.
