I was able to ask for an invite on a public forum, and I was able to use my demonoid (remember demonoid?) stats as proof back when they were semi private.
if you can show that you are a good and consistent uploader someone usually will sling an invite your way.
I would and for the most part have used private trackers for either a specific type of thing or a specific kinda obscure or not very popular title. I find that the more not well known a thing is, the more likely it’s going to be found and (re)seeded on a private tracker.