G1 has been able to do that for many years (for the much the life of the G1's existence afaik) using Magnet Links. Not so sure Shareaza's G1 tools have that ability. Well at least it can be done per file, not per group of files afaik.

With LimeWire 5 or WireShare there is a private sharing option that allows people who know each other to share a private selection of files. Although firewalling can be a deterrrent. They need to know each other's email address using either gmail, jabber or LiveJournal. Once the inital set-up, each time your program starts up on the network your friend(s) private share list will be browsed by your program and vice versa. Possible to have quite a few separate private share lists to share with different people. They can also text chat in real time privately. LimeWire had been trying to modernise not only the way the network could be used but also a more modern look option. I haven't tried using this function in a while so not sure if it still works.

I'm sure several file sharing networks share similar abilities except achieved in a different manner.