How many people can play?
The total is 3 players: you and up to two friends. You can also play solo.
Invite a Steam friend
Check the in-game Invite player option if you are having trouble with another join button or a public password room. Use the invite to bring your Steam friend into the session, then confirm that they have actually joined before heading out.
Check the lobby password
Patch 1.0.47.0 fixes a problem where removing the lobby password did not take effect. If that matches your problem, check that the installed update includes the fix and recheck the lobby’s password setting.
For platform support, see crossplay. For a specific joining symptom, use the multiplayer troubleshooting checks.