How did you get a private tracker?

All the most popular stuff is easily available on public trackers. For older/obscure stuff, you can run your own tracker easily enough that scrapes the DHT, although you'll probably burn through an SSD doing it. https://bitmagnet.io is one such self-hosted piece of software.

Personally, I got my first invite by signing up for a seedbox accepted by the tracker. Then I got invites to other trackers from the same group by being a good seeder.

You don't need a private tracker for stuff that comes out now.

In fact, for those things, I'd say a private tracker isn't that interesting because of the share requirements.

You find someone who is already a member and ask for an invite.