A neat way to check which invite-only bittorrent trackers is trackerchecker.org. It lists most of the open trackers, and if you sign up for an account, then they will automatically email you when the torrent site you want to join opens for signup.
They have all the popular trackers on their site, including demonoid (the following screenshot only shows a small handful of the total amount of sites in their index! You’d also be surprised at how many trackers are open right now.
Of course it’s probably easier to just get an invite from your friend