Rider to Rider Communication via Bluetooth

[OakLeaf]

Sena systems are well regarded among the long distance crowd, though I haven't done any B2B comm myself.

Bluetooth itself has a very short range - 10 meters or so - which means BT alone would work on a tandem or between rider/passenger on a moto, but wouldn't be much use for riders on separate bikes (unless you actually wanted to call each other on the phone). But some of the newer BT systems can also do "Universal Intercom" protocol with a firmware tweak, basically a walkie-talkie band, with a range of a mile or more.

AFAIK most of the headsets out there are designed for moto helmets, so you might need to modify them a bit to mount to a bici helmet.