Wi-Fi Direct messaging
Wi-Fi Direct links two Androids over the Wi-Fi radio with no router. In offChat this layer exists only if the device offers it.
No router, still local
Wi-Fi Direct does not use hotel or venue Wi-Fi. The phones talk to each other. It is still a local link, subject to distance and obstacles.
If Android does not expose Wi-Fi Direct, offChat can still use Bluetooth for people who are close.