Here is an example.
You will use the action Multiplayer: Broadcast Message for the host, and the condition Multiplayer: On Peer Message or On Any Peer Message for the peers.
Edit: I'm actually not sure myself how changing layouts interact with object syncing, maybe someone else who has experience regarding this can chime in.
Changing rooms is also an interesting idea, but I think re-associating peerIDs to the correct people could possibly cause a lot of headaches and problems. I assume the peerIDs will change upon entering a new room.