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.
Here are some private trackers that are currently open for signup:
DCtorrent.com : Desi Club Torrents - Tracks latest Bollywood movies, music and drama series. Status: Signups now open.
GFXnews.ru : Russian torrent tracker that tracks a lot 3D/2D graphics content. Status: Signups will be open from 02 jan 2008 07:00 GMT till 03 jan 2008 07:00 GMT
SuperTorrents.org : Popular site that tracks movies, music, appz and more. Status: Signups will be open on the 23rd of December.