Rhino's are the way to go if you're wanting to keep tabs on everybody. Once you beep everybody in you don't even have to be able to reach them on the radio and you can query their locations via the GPS, which is satellite driven not radio driven.
I got a couple so I can keep tabs on my dad a little better. He tends to push the envelope a little when he's hunting and I often don't know where to start looking for him. Good safety net for older, younger or inexperienced hunters (as long as they leave the unit on).
And they're water proof, not "resistant".