1. In order to play the cooperative mode, first you must finish the prologue. You should reach the quest called “First Assignment” (a step called “Sleep through the night”).
2. Remember to set the GAME TYPE option to PUBLIC in the ONLINE menu.
MAIN MENU->OPTIONS->ONLINE-> GAME TYPE -> PUBLIC
3. The game will inform you about being able to play in the coop mode:
To join or invite players to the cooperative mode, press the ESC key and choose MATCHMAKING.
The next step is to choose one of the available options:
QUICK JOIN immediately finds you a random game to join.
FIND GAMES shows you all available games you can join.
To join your friend's game:
Press ESC during the game and you'll see a list of all online friends (bottom right corner). To join their games, you need to click a player's name and choose JOIN.
Should you experience issues related to the multiplayer aspect of the game, see our other articles: Multiplayer-related Issues