F2F P2P

from http://en.wikipedia.org/wiki/Friend-to-friend

A friend-to-friend (or F2F P2P) computer network is a particular type of anonymous P2P in which people use direct connections with their "friends". F2F software only allows people you trust (using IP addresses or digital signatures you trust) to exchange files directly with your computer. Then your friends' own friends (and so on) can indirectly exchange files with your computer, never using your IP address.

These networks are also called private P2P though they can grow in size without compromising the users anonymity.