I guess I'm socially inept both in real life and on the Internet, because I never figured out, how do you get on a private tracker?

Depends on the trackers, most of the time you need an invite.

Private trackers are often topical, either focusing on movies, video games, music, ebooks etc... So if you want an invite to these you just need to hang out with people who share these interests and then ask around. IRC is a good place to start in my experience.

Some semi-private trackers also have periods where registration is open to all.

Some trackers have periods in which they have open registrations; there are a few sites which track these (like r/OpenSignups), so I subscribed to one and registered to those that seemed interesting.

Otherwise, you have to get an invite by an existing user; if you don't know any, I think the best bet is to lurk forums and IRC channels and beg for one. I never actually went that route, though.

Redacted (like the late What.CD, which it aims to replace) has interviews (https://interviewfor.red/en/index.html).

Most private trackers requires an invite (from someone who is already a member and has invite privileges) though.

What.cd was one of those - you literally had to get an invite from friends (inept as you that didn't work for me) or apply via IRC (can do that!), including an interview to make sure that you understood the rules and had a basic understanding of audio formats, encoding parameters.

People that brought you on were supposedly responsible for any problem you caused..