If they are offsite in a house, they will need a vehicle to get around, so a rental is going to be the way to go, rather than a transport service.
15's are available, but they can be difficult to drive if you are not used to something that size, and they are usually VERY expensive, as the usual rental market for them is one-day rentals for moves or as shuttles for special events. They also tend to have an uncomfortable ride; VERY bouncy. (My company has an Econoline 15 that we use for shuttling attendees at training sessions; I've driven it, and it's a tank.) One other thing; in the usual 15, the seats are stacked right up against the rear door; there is very little extra room for luggage, except under the seats.
My recommendation is to get two vehicles. It will very probably end up being much cheaper, and will give them some freedom to split up on occasion.