Stardew Valley co-op is a whole lot easier, thanks to the Stardew Valley 1.6 update. With online multiplayer and split-screen, you can play with friends and start your farm together as a team.
you should be able to see the game under the Join tab of the Co-op screen from the main menu. Do you already have a long-running Stardew Valley farm that you want to turn into a co-op game?