Solved co-op down

[Gingerpeacher]

cant join a game and the invite button is missing, has been playing with my girlfriend on steam.

 

[ShinyPorygon]

GOG server went down. Steam uses them for stardew as well. You will have to wait for GOG to fix it. No ETA.

 

[Odin]

It appears that GOG's servers are back up, this issue should be resolved now.