Private torrent trackers are smaller communities which cater a limited number of users. In contrast to large public trackers such as "The Pirate Bay" and "TorrentBox", private trackers provide faster download speeds and less number of fake torrents. However joining a private tracker often requires invitation from an existing member. If you do not have an invite, you can join through normal registration when the private tracker 'opens' its registration.
Instead of monitoring these sites 24/7, you can unleash the power of Google Search to find the ones that are open.
Type torrents inurl:signup.php in the Google search field and and hit search.
Note: Not all of the search results will feature open trackers but most will.