They are invite-only and are a completely separate channel. Right now this option is available only for partnered streamers (like myself - twitch.tv/anthonyacc). We are currently streaming and using the feature, so please come by and hang out!
"One of the defining features of Twitch is the ability to chat with fellow gamers. It's where community memes begin, it's where gamers express their glee, outrage, befuddlement, or amusement, and it's why the platforms' engagement level is off the charts. Twitch is a social place, and chat is where the social exchange happens."