Stardew Valley co-op was introduced in update 1.3, back in August 2018. Previously a single-player only game, Stardew Valley multiplayer adds a number of gameplay elements you won't find in the standard game, including the Four Corners Farm Map and the ability to share money between players; you can even marry another player instead of one of the 12 bachelors and bachelorettes. Here's all there is to know about how to play co-op in Stardew Valley.
How to play co-op in Stardew Valley
There's two ways to play multiplayer in Stardew Valley; either with a brand new world, or with an existing world converted from a single-player save.
In order to start a brand new world, from the main menu, one player must select Co-op followed by Host. Decide on how many cabins you want for the number of players (up to four) and alter the other options to your liking, then you're good to go.
On the other hand, if you want to use your single-player world in a multiplayer game, you need to build enough cabins for the amount of players you want to play with. When you have the right amount of cabins, you need to exit to the main menu, select Co-op, then select Load and choose the relevant save file.
For other players to actually join the game, the host needs to start the game then go to the settings. By default, all multiplayer worlds are closed to other players, so you need to open them up.
PC (Steam): If everyone you want to play with is playing through Steam, make sure you're friends with everyone first. Then your multiplayer game should appear in the list when your friends select "join".
PC (GOG Galaxy/Steam): If the above method doesn't work or you're playing on PC through GOG Galaxy, the host will be able to find an invite code in the game options. Other players can type in the same code on the join screen to connect.
PS4: Make sure you're all friends with one another on PSN, then you can connect to each others' games. PlayStation Plus is required.
Xbox One: Like on PS4, you all need to be friends on Xbox Live, then you can connect as long as everyone has Xbox Live Gold.
Nintendo Switch (Online): Just like on the other consoles, you can join the farms of people on your friends list via the join screen. You need a Nintendo Switch Online membership for this.
Nintendo Switch (Local): Enable local communications play and you can play with two Nintendo Switch consoles in the same household without a Nintendo Switch Online membership.
That's everything there is to know about how to play co-op in Stardew Valley. For more details on the features included, make sure you check out the Stardew Valley multiplayer wiki page.