I like the EF Johnson Viking VP 600
It would require buying a license, but if you used a digital mode - most no one else would be able to listen to your conversations.
Nothing that you can buy over the counter is going to be secure, and anything that you can buy over the counter - won’t be something that someone else didn’t already buy.
In a situation where there is multiple radios - chances are, the GMRS / FRS frequencies are going to be more busy then a woodpecker in a petrified forrest.
CB might or might not work, but the radios out there today are not durable and are not rechargeable while mobile.
You don’t have the time to get each and every group member licensed as a amateur radio operator and a subscription service license is probably going to be your best option.
Battery life will be dependant upon use.