Yesterday a brilliant idea came to mind:
Gee.... wouldn't it be great if there was an easy way to share files with friends, but not the whole world...
user level access...
hmm.... Google!!!

So I found FolderShare for Windows which is strikingly close to what I'm looking for - except it has no linux availability.

Does anyone know how I could set up a p2p server that requires authentication and allows users to specify specify which other users have access to which shares?

Bitwise is a chat program, which might suit your needs. it has the ability to share out a folder to anybody on your buddie list. I have used it with a friend of mine, and it works well.

you can get it from:

www.bitwisecommunications.com

click on "free bitwise" at the top to get to the non-buisness use portion of the site.

The stuff for sharing out a folder is in the preferences of the program.

This is also a great chat program in itself. not sure if it's in the stable edition, but the latest alpha finally gives me an icon in the KDE "sys tray"(not sure of real name)

NeoModus's Direct Connect is pretty close, I'd imagine. You'll have to look for Linux clones though.