You might want to start up an LJ comm--audiography does a brisk trade in mp3s via yousendit.com.

The reason I would advocate LJ comms over message boards--which are also good in different ways--is that you can restrict LJ comm membership to invitees only and make the entries members-only. If you're going to trade music within a dedicated group, you might seriously consider LJ. I don't know how you're doing the mixtapes, but using a comm might make it even easier to trade music, because I can't imagine Wing's been letting y'all link to downloads.