Invite only app maybe? If you get the sign up process to be an NFC or QR code type thing you could effectively limit it to people in that area that know each other or that see the qr code. You could even have the signin do imprecise GPS checking if you wanted to limit it further (not fool proof, but does it have to be?)

You could have something like the described in the digital bonfires idea and have a regularly scheduled means of moving things from local to people just passing through.