Even though Stardew Valley is a simple and relaxing game pretty much anyone can pick up, getting its multiplayer mode up and running has proven to be quite the challenge for the developers. Thankfully, after many months of work they now appear to have solved most of the issues given that the much-requested co-op multiplayer mode has finally been confirmed for an early 2018 release.
Once it arrives you will be able to invite up to three other players to live on your farm, each with their own little cabin to call home. The other players, or rather your farmhands, will be able to do pretty much everything you can: farm, mine, fight, fish, forage, marry NPCs, take part in festivals, and so forth. The only things your fellow players won't be able to do is dictate 'large' decisions such as when to sleep, when to start and end festivals, and naturally, whether to side with Joja or not. Oh, and if you are so incline you will also be able to marry your companions, though the exact details behind that process haven't been worked out just yet.
Stardew Valley's co-op multiplayer will be coming in early 2018 for PC, Mac, and Linux, while the consoles will be getting Update 1.3 a little bit later. As of right now local multiplayer, split-screen, and PvP are not planned, but hopefully the developers will reconsider the first two as Stardew Valley would be a pretty damn fun split-screen game to mess around with.
I've covered pretty much all of the important information, but if you're interested in all of the details you'll find what you seek on the official website. And finally, here's two more multiplayer screenshots: