Private trackers don't really do much to improve P2P security, nor will they stop you from getting angry letters from your ISP. If you, a relative Joe Nobody, can get an invite to a private tracker... You really think a multi-billion dollar industry can't get an invite too? They have entire teams of people whose entire job is getting accounts on private trackers and then quietly reporting back to copyright owners while they act like good little users.
Use private trackers for day one uploads, better upload quality, curated content, more reliable seeds, an established request system, etc... But don't use them for security. Because the P2P part works exactly the same as a public tracker, which means anyone connected to the swarm can see your IP. Use a VPN if you're torrenting.