For server use groupchat.google.com.
For room, you need to create a UUID by using an offline or online utility, then enter private-chat-UUID as the room name (obviously replace UUID with the actual one you generated).
Source
Edit: To invite others to your newly created private chat room you'll need to send them the exact same room name and server you used (confirmed to work here). Unfortunately, as long as Google Chat in Thunderbird lacks an Invite option this roundabout way seems to be the only solution.