The game owner (crown next to name) is player 2, and has to be spectator.
The person in the first slot (top left) of the lobby is player 1.
Use workshop settings to decide how player 2 can control player 1 or how many buttons player 1 can still use. Player 1 always has movement and aim control.
Due to workshop limitations, player 2 has a few weird controls. Jump is bound to crouch and Secondary is bound to Z.
Reminder: Right click to spectate someone and space to toggle 3rd/1st person.