It will be the entire map, there will be a tether range (limitation for playing as a server and a client at the same time), and you do have to stay on the same grid as one another; only the host will be able to initiate a transfer as the host is the server, so they control which grid is active or not.
We working on this too, it's currently being tested by our team so hopefully, it'll be good to go with the release of the game mode. Though you'll want to sail in tandem with the host in order to make sure you're not pulled off your boat for exceeding the tether range. It's recommended that you slow down once you get towards a transition wall, so that everyone can be together rather than sailing off into the distance.
Saved locally in the SavedAtlasLocal folder. It's the same as the SP save.
devtrackers.gg