You can watch your friends’ games with the click of a button via Steam Broadcasting, now available in beta. When you see a friend in game, simply click on "Watch Game" from their profile or from the Steam Client Friend's List to open a window into their gameplay -- no game ownership, special fees, or additional apps required.

The beta is open to everyone on Steam who wants to participate by opting in to the Steam Client beta through the Steam Settings panel. For the time being, concurrent viewing may be limited as the beta is scaled up to support the broader audience.
(View => settings => BEta participation => restart steam).

We hope to see you soon broadcasting !