E.g. iptorrents.com. You pay for access (or have to be invited) to a catalog of torrents that are not publicly-available. You must disable the DHT and peer discovery features of your bittorrent client so that the file content is only seeded back to other members

The better trackers are invite only. If you are found to be selling invites your entire invite tree will be banned. You have to be recruited or personally know a member to get in.

Take for example PTP. It is one of the best movie trackers. It recently exceeded 250,000 unique titles available in many different formats. Netflix in contrast only has 15,000 unique titles and their library is shrinking not growing.