Not really much to say. Twitch sends emails about the top events that are happening, and the first channel shown when selecting the channels button shows the most viewed channel (usually I've seen it in the top 20ks).
Just hop into a channel when it have over 100k viewers and this will unlock.
*I got the achievement from joining RiotGame's stream (League of Legends developers). So chances are, if they're streaming you can unlock this achievement.
*As per Arsenic 17, xCUBBYxxxBEARx, and John Caboose's comments, you may have to have the app snapped when you join the stream in order for the achievement to unlock. The app for some reason may not track how many current viewers there are in full-screen mode, so make sure it's snapped. If the achievement doesn't unlock when you know it should, quit out of the stream and either un-snap or snap the app and try to join the stream again.
*Seems like if the stream has more than 199,999 viewers, this will not unlock.