The software urges people to not invite buddies that they did not know on a medium-term, face-to-face basis. You could also allow certain people to share your files without letting their friends share your files.
update checker - software will prompt user to update it when new versions are available so users won't get stuck with an insecure or broken version.
Unless, of course, the FBI/RIAA seize/hack the server and put up their own updates which sends lists of evidence home; encrypted, of course, and through a P2P network to disguise the telltale fbi.gov destination. The first most users will know about it is when they're rounded up in a series of dawn raids and the Attorney-General goes on CNN announcing that the FBI have cracked a major international piracy (or child pornography) ring.