Need help figuring out how to make a singleplayer experience multiplayer with an object interaction

Hello, I’m trying to wrap my head around making a multiplayer system. The system needs to be laid out where the “Host” is in their own world already and Opens their world to others joining by interacting with an object.

Now, would I take all of the saved objects in the world and pass them into a new level with ?listen enabled?

I want people to be able to take their local save game and pass that information into the hosts game so they would have all of their items and skills. Does anyone have any idea how I would go about doing that?

Thank you in advance.