That’s misleading. There is a “message” that is sent between clients to ratchet the group forward and provide forward secrecy between epochs (when the full ratchet tree is refreshed), however the user doesn’t have to think about that at all and happens on a regular basis in any normal 1-1 or group chat. 

There is also another layer of forward secrecy that is provided by the message keys themselves (basically using the same mechanic as Signal).