We have a customer at a campground which has a Merlin Legend with Merlin Messaging. They want the campers to have access to voice mail so their parents, etc can call and leave messages. Can a group of voice mail boxes be assigned for the campers without them having access to non campers voicemail?

That's what passwords are for. Set the minimum password length to 0 digits, tell the campers to dial 770, their mailbox number and then "#-#" to log in. The non campers should use strong numeric passwords.