Developer ConcernedApe just announced that a PC beta of the co-op mode for Stardew Valley is now live and can be accessed by fans via Steam by doing the following steps:

First, players can enable the new beta by right-clicking on the game in your library and select Properties. In there, you’ll find the Betas tab and type the password “jumpingjunimos” and click the Check Code button. Once done properly, players should be able to access the Beta in a drop-down box, which will automatically prompt the game to download version 1.3. As always, it is better to back up your game first before downloading the beta, as advised by ConcernedApe in Steam. 

The multiplayer mode of Stardew Valley allows players to have up to three friends living on their farm. Of course, your friends will need a place to stay in so there is an option to build them cabins or use pre-built ones. The good thing about this is that Cabins are not that expensive, so it's easy to have your friends with you early in the game. All three players will share the farm, money, and world updates. However, inventory, skills, and relationships with the villagers will be kept separate. 

The new co-op mode will also give players the option to choose whether you want your friends on Steam to join, or if you’re a bit picky and would like your farm to be strictly by invitation only, which would require players to input an invite code to join the server. Furthermore, ConcernedApe revealed that players could marry each other. However, only the host player can pause the game, and sleeping would require all players to say Yes to a dialog box for the day to officially end. 

You can check out the full patch notes on Steam. ConcernedApe also revealed that new content would be coming to the single-player mode soon and that multiplayer mode will also head to consoles, but the Nintendo Switch will get it first.