Finally… Google offers the ability to do group chat by way of the Google Talk Gadget. It’s not available with the client version of Google Talk, though.
To use it, start a conversation with a contact, click on Group Chat and select additional contacts.
To have concurrent group chats, even involving the same individuals, repeat the above instructions.
In Firefox, if you bookmark the chat url and select the option in bookmark properties to load in sidebar, the gadget will open in the sidebar so it’s always available.
I have added a Chat button at the bottom of my sidebar, with a username to add to your contact list. This is an experiment to see how useful this could be. For example, if someone needs help with code that I have provided, or have questions that might be more quickly addressed via chat than email, then that is an optional method of contact.