The game supports seamless drop-in/drop-out co-op for up to four players. You can manage sessions through the in-game menu:
: Both players must have the latest patches installed to ensure connection stability. Key Survival Tips
If you are just starting out with a group, keep these strategies in mind from the Generation Zero Survival Guide :
: Launch your game and navigate to the Multiplayer section in the main menu. From there, you can choose "Invite Friends".