How a session is hosted
The published FAQ describes co-op as peer-to-peer: one player acts as the host when friends start a game together. It is not described as an always-on public world.
Plan around the host world
Agree who hosts longer sessions and which world carries shared story and building work. Players can bring a hero into another world, while world-side progress remains associated with that world.
Before inviting a group
Check each player's version, connection, and availability. If the live release changes hosting options, follow the current in-game menu and official notes.