This is what you probably already know but honestly, the best way is to know someone with an invite and if you don’t know someone irl, try to get involved in an online community.

Another way is to watch for open sign ups. Some private trackers have applications, like MyAnonamouse. Once you’re in, check out the forums for invite requirements to other private trackers. You often need to climb a few ranks to gain access to the invite sections of the forum.

Best of luck to you!