The short version
Before a co-op session, agree whose World is the campaign home.
Solo or 1–8 players
Dimraeth supports solo play and online co-op for up to eight players. The public FAQ describes a peer-to-peer host model rather than dedicated servers.
Character progress travels
Your character, attributes, equipment and personal progression are the portable layer. This is the layer you should prepare before joining another player's world.
World progress stays with the World
Story Progress, Buildings and Recruited Allies are tied to the host's World. Treat a friend's save as its own progression track rather than assuming it advances yours.
Choose a host deliberately
For a shared campaign, let the player who wants to own the settlement and story route host the World. Everyone else can bring their character into that world.