Steam Remote Play Together Beta

Valve announces the promised open beta testing is now underway for Steam Remote Play Together, their recently announced method to allow online multiplayer support for games with local multiplayer. This post has details on how to utilize this, and the announcement has an overview, including a reminder that only the host needs to own the game being played:
Steam Remote Play Together is now available in beta. With this latest feature, you can now invite Steam Friends to join your local co-op, local multiplayer, and shared/split screen games online. Up to four players, or even more in ideal conditions, can instantly join in the fun. Only the host needs to own and install the game, while additional players connect through Steam Remote Play streaming technology.