One of the most exciting features of Stardew Valley when it was first announced was the prospect of co-op, but Stardew launched with multiplayer still an on-the-horizon feature. And it's still coming, with a planned beta in early 2018. In the meantime, there's a surprisingly easy alternative thanks to modder spacechase0. 'Makeshift Multiplayer', which was last updated in October 2017, does exactly what you'd hope.
The mod allows you and your friends to join a single game of Stardew Valley to play together online, and it actually works. You share a wallet, relationship progress, and a few other things, but you have separate inventories, skills, and even different farmhouse interiors on the other side of the same door. The mod isn’t perfect and there are a fair number of bugs, but it's the best we have for now. It's a fun way to co-op Stardew Valley with a friend.
The instructions below may look intimidating, but it’s actually surprisingly simple to get working—especially considering the mod is adding networking to a game that previously had no online functions whatsoever. Plus, it’s a great way to get introduced to other amazing Stardew Valley mods.
How to play Stardew Valley multiplayer:
All players will need to install both the mod and the Stardew Modding API (SMAPI). Additionally, you will either need to be connected by LAN or the host will have to make sure they’ve forwarded port 24644 on their router before anyone online can connect. (That's the default port; you can choose another if you prefer).
Step 1: Download SMAPI, unzip the folder and place its contents in your root Stardew Valley install folder (C:\Program Files (x86)\Steam\steamapps\ common\Stardew Valley by default) next to the game’s normal exe.
Step 2: Download the Makeshift Multiplayer mod, unzip the folder and place its contents in the “Mods” folder that came from the SMAPI zip.
Step 3: Launch the game from “StardewModdingAPI.exe.” You will still be able to play the unmodded version by launching the game regularly.
Step 4: For the host, make a new character or load the game you’d like to play in. You can launch the multiplayer mod by clicking Load, which will let you choose your character, and then choose from Singleplayer, Host, and Client options. Choose host and share your IP address and port with the client.
Step 5: For all other players, you're clients. Wait until the host has done the above step, then make a new character or load the one you’d like to use. You can launch the multiplayer mod by clicking Load, which will let you choose your character, and then choose the Client option. You can select your host via the friends list embedded at the top of the screen; we had more success with this than manually typing the IP. Click their name and wait, or type the host's IP and click connect. Wait.
Step 6: The host should now see a pop-up saying a player has asked to connect. Hit accept, and give the client (or clients) some time to connect before hitting Start. You can look in the SMAPI log window to see that this takes a few seconds. If you press Start too quickly, it'll break things, so wait until you see this:
[Makeshift Multiplayer]  yourfriend has connected.
Once everyone's connected, hit Start to begin!
Step 7: Wait. It may seem like something's gone wrong, but it can take about a minute for the online session to get going. Go get a drink.
If you are having issues, you can check the 'Posts' section of the Nexus Mods page here to see if others might be encountering the same thing.
There are definitely bugs and problems you should be aware of. Other players can get stuck in an awkward run animation, and when one of you gets pulled into a cutscene the other person can still run around like normal. Additionally, don’t expect holiday events to behave correctly with multiple player characters, so it might be best to play through those days alone and then have the other players rejoin you after.
Another big one: time is only linked to the host player. It pauses when they are indoors or in a menu, but keeps on ticking even if others are paused, so you’ll probably want to coordinate to make the most of each day. You could even intentionally keep the host paused while the other players get some extra work done, but where’s the fun in that? And remember, you all have to make it to bed to end a day—don’t let that one party animal in your group make you all stay up until 2 am.
Even in a rough state, Stardew Valley co-op is pretty fun and gets me excited for the real deal.