Hi,
As promised in the last update, we've added multiplayer online to the game in the beta branch. In order to select it, you have to go to Preferences->Betas and pick the beta branch instead of the default one.
The feature is in a beta instead of the default version because we're not sure how stable the game is online or what bugs you'll find. We've done our best to try to minimize the bugs but we're sure there'll be many we haven't seen or guessed. That's why we'd like to ask for your help. If you see any bugs or would like to suggest any improvements for this game mode feel free to go to the forums to voice your opinion.
We hope you enjoy the feature, we know it's been requested by many of you.
Thanks!
Eduardo