Radios / walkie-talkies / other ride-day live communications solutions is the subject of extensive discussion. Bottom line, don't skimp. If a ride is tiny to small just yell or use phones. At the 150-500 level GOOD walkie talkies are a BIG help. Buy more than two of the highest power and distance allowed by law. At the 1000 plus level we're debating PTT rentals as the only reliable for distance.
More details will be posted here when finalized.