http://www.theguardian.com/world/2014/sep/29/firechat-messaging-app-powering-hong-kong-protests

The Bluetooth range is really short, though newer versions have more power and longer range.

Presumably there's some way to get WiFi to do the same thing - in principle. Dunno if such a thing actually exists, though.

HTH.

Cheers,
Scott.