Another option: goTenna mesh. You need a cell phone to use it but you do not need service. Note that it is text messaging only. Messages can include GPS coordinates making it very easy to figure out how to find someone. In my limited experience with it I think it does what you want and note that the units will act as limited relays--if A can reach B and B can reach C then a message will get from A to C even if they can't directly talk.
I'm not sure of the legalities in other countries.