Alone or together
Every part of the game works solo; with friends it is two to four players on a server that owns the world, each with their own loot.
Art in progressEvery part of the game works solo. Playing alone runs the same server on your own machine, so solo and co-op are one game and nothing has to be built twice.
With friends it is up to four players. The host's game runs the world and everyone else's mirrors it, so what you see is fair for everyone, and your own movement is predicted so it feels instant. Experience goes to every living player within 30 tiles of a kill, the portal takes the whole party down, and a fallen player gets back up at the start of the depth. If the whole party falls, the depth starts over.
Loot is instanced: each player rolls their own drops and only sees their own, so loot never decides who is a good friend.
One coal dies; a heap of them burns till morning.
You can host from the game, run a dedicated server, or join by address. Friends over the internet need the host to forward a UDP port; there is no matchmaking or relay yet. Co-op in the wiki has how to host and join.