As per Steam’s announcement, you can now invite your Steam friends to join your local co-op game, effectively playing traditionally split-screen games online. It supports up to 4 players, and only the host needs to own the game. Additionally, this means that you can play with your friends regardless of whether they have PC, Mac or Linux.
To get started with this exciting new feature, you just need to join Steam Client Beta and then launch the game you want to play. Afterwards, you can access your friend’s list on the Steam overlay and invite them using the option Remote Play Together, which they would have to accept.
And that’s it, you should be playing the game together, the invited friend or friends will be able to stream the game directly on their screen and their keyboard, mouse or controller should work to play.