If you are already on some private tracker: https://inviteroute.github.io/graph/

Join PTP, RED etc and get access to the invite forums. FYI you won’t get an invite from a regular user, it takes years to get to the user class that gets invites

you either get invited by someone you know or you build your reputation on a different private tracker and gain access to their invite forum where BTN have an official invite thread.