I have been taking whistles on trips with me for so long that I take for granted that not everyone does. A whistle weighs next to nothing and can be extremely useful in the wilderness in a normal situation, let alone in an emergency. It is nice to be a good way away from your friends on a hike and be able to give a short whistle blow to let them know you found something interesting.
Before you go on a hike, give everyone a whistle and have them attach it to a zipper pull or somewhere else where they are sure not to lose it and will always have it readily available. Work out a system of whistle blows with them, where 1 long blow means ‘come here,’ 1 short means ‘I’m over here,’ 3 short bursts means ’emergency,’ etc. Keep it simple. You don’t need to invent the next Morse Code. The usefulness of a whistle is defeated if someone can’t remember what 3 long, 2 short, 1 long, 4 short means.
I tend to use whistles in place of walkie talkies. They are lighter, don’t require batteries, are less bulky, and are easier to have always ready. They, of course, do not have the range or depth of communication that a walkie talkie does, so don’t replace them if verbal communication is essential.
One of the best, lightest, and smallest whistles on the market is the Fox 40 Rescue Howler. It is tested at 122 decibels, which is plenty loud for anyone nearby to easily hear you. I have been using one for years and just picked up a pile more at the REI super clearance. Maybe I’ll give some away to the next few people that post some good, detailed advice topics.