iOS version ("Solidarity" in App Store): http://itunes.apple.com/app/id956548749
Solidarity is the app that synchronises your state with the crowd.
It works without Internet access by connecting phones directly through Bluetooth and infrastructure Wi-Fi.
Please note that for Bluetooth connection to work, one of the phones must have Android 5.x (Lollipop or newer) AND hardware support for Bluetooth LE Peripheral mode (have "yes" in the last column here: http://altbeacon.github.io/android-beacon-library/beacon-transmitter-devices.html).
Over Wi-Fi app is compatible with iPhone version.