Something i have carried and used to get the attention of groups spread out over large areas is a horn, the type you used to see on old cars, a horn with a rubber bulb that you squeeze. Modern ones of these are small, strong and light. I first came aware of them seeing demolition teams use them to warn that they were about to detonate when dropping chimneys and other structures. the cave rescue guys at my caving club used these to communicate too. I must replace my horn as it went walkies.