We now have a better idea of when multiplayer will be coming to Stardew Valley and how it will work when it does. It turns out 2018 is the target for the Windows, Mac, and Linux 1.3 update. However, a multiplayer beta test will be run through Steam at the end of 2017. Console versions of the games will also get the multiplayer patch in 2018, with the Nintendo Switch being the first system to offer co-op.
Adding friends will involve having Robin build farmhand cabins on your land. You can have up to three, and each one can be controlled by a friend over the internet. Farmhand characters can do almost anything your main character can do, including marriage, participating in festivals, and various farm tasks. In fact, the main player can actually marry a farmhand character, with its own wooing system and alternative to the NPC mermaid pendant. The main player is the one who determines when a day ends via sleeping, when festivals begin, and whether that game will side with the community center or Joja Mart. Also, when a farmhand is not playing with you, you can use a chest in their house to manage their inventory.
So far, 15,000 lines of code have been rewritten as multiplayer is implemented in Stardew Valley. Future work involves implementing farmhand cabins, adjusting UIs and menus, and working on multiplayer NPC relationships, events, and festivals. After that will come testing and perfecting the experience. Only online multiplayer is planned, and local, PvP, and split-screen are not currently being considered.
Stardew Valley is available for the PlayStation 4, Xbox One, and PC. A Nintendo Switch version is in development.