You need to set up the connection first.
Take a look at the templates with the multiplayer templates to set up a working connection.
After that its a matter of choice, either synch all the objects you want with the multiplayer object synch (this can be heavy on the lag if you synch too much) or loop through the objects to be synched once a peer connects, and sent their states (asjson) over to the peer, which catches the messages and sets the objects accordingly (create them if needed)
After the initial synch you could broadcast object states/changes in messages to peers.