https://steamcommunity.com/updates/broadcasting
To try out Broadcasting now, both you and your friends need to opt into the Steam Client Beta. In the Steam client main menu, select Settings > Account and change your “Beta Participation” to “Steam Beta Update”.
How do I start & stop my broadcast?
When you’re playing a game and someone starts watching, you automatically start broadcasting. When you end the game your broadcast ends.
Hotdamn