Today Twitch introduced a new addition to the Friends feature – Activity Sharing with Friends.
Activity Sharing allows you to share what you are watching or broadcasting with your entire friends list. You can also easily join them by clicking their status under their username.
Your friends list will not only show you who’s online, but also show you what everyone is up to.
Activity Sharing is on by default, if you do not want to share your activity with your friends you can simply opt-out by clicking your own status under your username.
via Twitch Blog