Hey Cjm,
Other players can join in the game by using gamepads. You can use any mix of controllers from playstation, xbox, and logitech. The keyboard and mouse acts as its own controller so it is also possible to play two players with only one gamepad.
Theres still a few bugs in the multiplayer I'm working on them right now for the update, Please let me know if you find anything
Thanks a lot,
Rafi