There is no kind of vetting at the campfire to assure that you are able to attend. The only hitch that a non-Disney resort guest might encounter is that to get to the campfire one would normally use a Disney bus. Well, access to Disney transportation is reserved for Disney resort guests only. In the unlikely event that the bus driver asks for your Key to the World card, simply exit the bus and hoof it. It can't be more than a half mile. But I sincerely doubt the driver will give you a glance. Enjoy the campfire!