The following tweet was posted not many days ago By Thomas Mahler:
I understand this tweet as the multiplayer aspect will be somewhat restricted. Here are my thoughts on the matter:
It would make so much more sense, and be much better, if worlds/realms are something everyone (except the host), can just join/leave whenever they want to with their characters.
This way everyone can play and trade with each other.
I think it would be a big mistake if the system prevent players from leaving realms (multiplayer) once they joined it.
It would mean that if your 3 chosen friends stops playing at some point, then you can never invite new friends into your realm, which could leave a potential endgame/developed realm to be a 100% singleplayer experience.