private bittorrent trackers. The notable difference, though, is that most content on SR is a quite a bit more illegal. Still, it's worth the try especially after all the heat from the assholes at Gawker.
Private trackers are about keeping seeding ratios up and preventing large influxes of leechers at one time. It's not even stopping the copyright police, mostly because they don't have to worry about it, can harvest there 10,000 IPs for the day's shake down laters from non-private trackers. It's not keeping law enforcement out. You think an invite only system would work for something that actually has law enforcement interest like a CP tracker?
There are two levels of security -- one relies on a small and close circle of trust where people are on a need to know basis and everyone is being vouched for. This only works in very small situations. Even then, not foolproof.
Many users - better security through obscurity. every small barrier you put up creates more problems for regular potential user, not a problem at all for someone whose job it is to circumvent them.