Chucklefish said earlier this year that the multiplayer update to Stardew Valley that everyone's looking forward to would not be out this summer, and sorry to say, I'm not here to deliver any happy surprises—that still holds true. But work has progressed far enough that the studio is now ready to start talking about how the new feature will work and what it will include, and said that beta testing is expected to begin around the end of the year.
Shortly after the game starts, Robin will offer to build up to three cabins on your farm, each of which will serve as the home for a farmhand who will be controlled by another player. Farmhands have their own separate inventory and can do "almost anything" the main character can, including farming, mining, fighting, foraging, marrying NPCs, and taking part in festivals. Some decisions will have to be made by the main player, however.
Friend invites will be handled through Steam, rather than separate multiplayer servers, and local multiplayer options, split-screen, and PvP aren't currently planned. Marriage between players is in the works, although how exactly that will work is still being hammered out. "It’s an idea we like a lot, and want to make available as a feature," Chucklefish wrote. "Player-to-player marriage won’t use the mermaid pendant, but rather an alternative method that requires a similar amount of effort to wooing an NPC."
The work so far has been focused on the "biggest technical hurdle" of synchronizing multiple games over the internet, and it has apparently been a huge job: Chucklefish said that 15,000 lines of code have been rewritten, and almost every source file has been impacted. But with that aspect of it almost finished, the work now will move to implementing the cabins, updating the UI and menus, figuring out multiplayer NPC relationships, and the usual "polish, testing, and optimization."
"We currently expect to be able to start a beta test at the end of the year for Steam users to help us test the game. Mod authors will be encouraged to update their mods during this beta period," Chucklefish said. "Then, in early 2018 we will release it as the 1.3 patch on Windows, Mac and Linux."
The will be released for consoles as well, but the studio warned that it "will be a bigger patch than usual," and so will take awhile to get to each platform.