To play online with friends, you can either host a private "Listen" server from your own PC or set up a dedicated server for 24/7 availability. How to Start a Private Co-op Game
Choose the maximum number of players (up to 4 for standard co-op).
: Once in the lobby or after loading into the world, right-click your friend's name in the Steam Friends List and select "Invite to Lobby" .
: After your friends join, hit Enter to open the chat window, click the two-person icon , and invite them to your team. This allows you to build and share base resources together. Dedicated Server Options
: Click the "Create Server" button at the bottom of the screen. Configure Settings : Set a Server Name and Password (to keep it private).