Create an Invite Only session
- Select Host Game on the main menu.
- Enter a save name. Change Session Type to Invite Only and choose the difficulty.
- Select Create and wait for the world to load.
- Pause and choose Invite Friend.
- Send a Steam invitation; the guest accepts it to join your session.




Find the guest-side entry
Join Game is a separate main-menu button. Its panel contains Join By ID and Find Steam Friends; the recorded example is version 1.0.5.
Private sessions introduced in patch 1.0.5 are invitation-only. Changing session type inside Server Settings requires restarting the session.

Retry a failed join
Update both copies and try a fresh invitation after restarting the games. If you changed session type, restart that session before testing the invitation again.
Check the rules before fighting together
Patch 1.0.4 raised lobby support to eight players.
Since patch 1.0.11, your own explosives can kill you even when friendly fire is disabled.
Agree who carries the current quest item before boarding, then use the island route to confirm the next hand-in.
Sources and further reading
How to Play with Friends in How to Fish | Multiplayer Tutorial Guide
How to Fish How to Play With Friends