We are thinking of asking our DS(10)'s best friend. He is from England and of course is not a US citizen (our family is from the US -just living in Canada as expatriates). Has anyone done this and what would we need to bring along for him? He does have a passport - so I'm thinking just a letter from the parents naming us as his guardians for that week? Should it be notarized? Also, how late can we actually add him? Sailing Oct. 19.