Channels can be set so that only users who are "invited" can join. In some cases its done to limit a channel to only friends, in others its done on a temporary basis to keep troublemakers out.
Considering that TMD channels are notorious for illegal filesharing of movies, i'm not surprised they set themselves to invite only. (yeah, like that'll help hide what they're up to *snort*) Take some advice, go rent the movies you want to see. Takes less time, is legal, and they wont infect your puter.
