The Forest just released the co op patch. For now it's just 2 players but they hope to increase that in the future.
Version 0.09 Changelog:
Slowed down tennis ball throw massively so players can play catch together on beach
(multiplayer)Server configuration screen - This will automatically tell you your public ip address - This is temp, server browser or better Steam integration coming soon!
(multiplayer)Chat box - Press enter to type messages to each other - This is temp and may be buggy!! Voice chat is coming soon! (we’re modeling walkie-talkies)
(multiplayer)Player texture variation, run around with your survivalist buddy!
(multiplayer)First pass at player respawn system, on dead try and find way back to body to collect your belongings
Fixed bug where building a bench could cause save games to break
Fixed female skin being too dark
Rest of skinny animation variations added
Saving in a multiplayer game will save over single player save game, allowing you to continue progress but not as a multiplayer game (working on fix for this currently)
Destroying partially built items can be glitchy. Try not to destroy structures built by other players.
EDIT: Yeah the update kinda made the game worse to be fair haha, co-op is apparently buggy to the point of being unplayable also.