I think integrating with Gnutella is a quite nice idea.
But it might be a good deal of work, to introduce the circle-idea into a Gnutella Client, except if you store the info about the files on your page and only use the Gnutella client as light-weight fileservers.
It would be partly possible to do that with private networks from Phex, but only by introducing mostly static groups which people can join and leave, and I think that isn't what you want to do.
But I have one interesting read for you, I think:
http://lionshare.its.psu.edu/ - Gnutella based P2P for Universities, which features groups and even integration of web-based sources.
PS: Sorry, that I didn't answer earlier.
I assumed your question had already been answered and didn't read on...
I hope my answer is still interesting to you.