For steam users, you can now play the multiplayer beta right now. Just follow the below instructions:
To access the Steam beta, find Stardew Valley in your game library, right-click it and select properties, then go to the "Betas" tab in the pop-out window. Enter the beta password "jumpingjunimos" and click "Check Code," then use the drop-down menu above the code to select the beta. Steam will automatically download the update from there.
For GOG users, you're not getting it for "a few more days."