Co-op opens once the prologue is over, and a session holds up to four players. Open the pause menu, then press the same button again for the online menu. There, Quick Join puts you in a random game, Find Game lists open sessions with each host's current quest, City Alignment and number of players, and you can invite a friend instead. PS4 and PS5 players can play together, and so can Xbox One and Xbox Series players (since update 1.9.0); don't count on PlayStation, Xbox and Steam players joining each other.