The coolest part is that only the host needs to own and install the gam; other players connect through Steam Remote Play streaming tech. The steps to joining the beta required to try out the new remote play feature include registering for the Steam beta. There are instructions on signing up for and downloading the Steam beta client can be found here.
Once registered, players can invite friends from their Friends list in the Steam Overlay, select Remote Play Together, and once they accept, players are playing together. The remote player controllers will act as if they are plugged directly into your computer. All voice chat and game volume options are in one place.
Steam says that only the game is shown when a game is hosted. The other players never see the host desktop or other computer content. Hosts can choose to share access to their mouse and keyboard, and Steam has a number of games listed that already support multiplayer. Another cool aspect of gameplay is that Remote Play Together is cross-platform supporting PC, Mac, and Linux systems for the same shared session.
Steam promises that during the beta, it will improve network stability and compatibility across a variety of hardware. The first time we heard about Steam working on the feature came earlier this month.