Ah, finally, a way to legally upload a ton of pirated songs and share login credentials so that hundreds of people can download them without hitting up torrents and thus placing 90% of the cost onto amazon.
You can of course already do exactly what you propose with various other services, though again many would shut you down if half the internet tried to access them for equally pragmatic reasons. Small darknets based on social networks are wholly possible though via nearly anything that can transfer files e.g. gmail, google docs, dropbox, IM, samba, apache, swapping USB drives, etc.