The confusion: The Steam store page still says 1-4 players. After Patch 1.0.4, the game actually hosts up to 8. You do not need a mod for 8. The cap is already in the live build. What trips people up is session type, Steam Overlay, and mismatched patches, not a hidden slider. The game is still tuned for about 4 players, but it will accept 8. Expect more lag, more seagull theft, and more people shooting the same fish.

1How to Host an 8-Player Lobby

2How Friends Actually Get In

If you hosted Public
Join Game from Steam Friends
Friend opens Steam Friends list, right-clicks the host, hits Join Game. They can also use Find Steam Friends in-game. If Join Game is missing, your session is Invite only or you are not showing as in-game yet.
If you hosted Invite only
Send invites from in-game
Host opens the in-game menu and hits Invite friends. If that list is empty, Steam Overlay is off. Enable it in Steam Settings, In-Game, or press Shift + Tab in-game and invite from the overlay. Joiners cannot self-join Invite only.

Session Type and Player Count Decisions

ChoicePick this ifDo not pick if
PublicYou want 5-8 people to click Join Game themselvesRandoms or griefers joining (rarer after 1.0.5, still possible)
Invite onlyClosed friend groupYou expect people to join from the friends list with no invite
4 playersYou want the game as designed: less desync, better money per catchYou specifically want the 8-person circus
8 playersBig Discord or stream stackHost PC is weak or anyone is on a bad connection
Players treat 8 as "possible and funny," not "better." Several reports: 8 is chaotic and half the lobby just stands around. Still worth it for the bit.

3How to Split Jobs in an 8-Player Group

Split jobs or you starve. More players split the value of each catch and fight over the same fish. Do not all dump damage on one target unless it is a boss.

1-2
On the boat
2
Fishing
2
Finishing / shooting
1
Hauling fish to cook/sell

Common Mistakes to Avoid

If someone still cannot join after Public, Overlay, and same patch: host remakes the lobby from the main menu and invites one person first. If that one works, add the rest.