You will have to store the data in the server. Every change a player make, will have to be sent to the server, and then re-sent to all clients. You can use the same principle to send the data to the new player that connects.
That data you send could for example be a json string, containing the important map data that you use to create the scene. You could create an "map" instance, that contains a function to update it, which is sent by the server upon connecting.
I did this with low level language, have yet to try this with the new high level networking.