Paid torrent trackers don't really exist as far as I know, that's only really a thing on Usenet. The most common way to get into invite-only private trackers is through recruitment threads on other trackers, so if you aren't already a member of at least 1 private tracker it's hard to get into the scene to be honest. There are a few private trackers with open signups, or an ability to apply for an account and get manually approved, but those are pretty rare. The only one I'm currently aware of having open signups is pornolab, and as the name suggests that is a tracker dedicated to porn only. Another one that is very easy to get into is IPTorrents. It's invite-only, but they are giving so many invites to members that there are lots of people that will give you an invite if you just ask nicely