Update, April 30: Stardew Valley's multiplayer update is now in public beta.
Been waiting for Stardew Valley multiplayer? You can play it right now. After some time in internal testing, the multiplayer beta is now open to all players on Steam, providing the full suite of options ahead of the new update’s full release. But do mind the (potential) bugs along the way.
Find out more about what's coming to Stardew Valley in update 1.3.
You can opt in to the beta 1.3 update by opening the game properties on Steam, hitting the betas tab, and inputting the code “jumpingjunimos.” That will force the game the to pull down the update and let you start farming with your pals. The beta will also go out to GOG users in a few days.