We used to own a Grand Caravan. As long as it is a Grand and not a standard Caravan, you should have enough room behind the 3rd row to stand the luggage on end and turn it sideways, thus lining them up behind the 3rd row. You should be able to line up about 4 suitcases this way and pile some duffle bags on top of them. If you have 7 people, you can pile a few things on the seat between the 2 in either the middle or 3rd row. You can line up 2-3 bags between the end of the middle row and the sliding door. People can hold carryons in their lap and possibly get some small things on the floor between their feet.
Are you staying on property? If so, you could have a couple people take DME from the airport to the resort, freeing up more room in the car for luggage.
If you're not staying on property and it all doesn't fit in the van, it would probably be very expensive to try to rent a bigger van then and there. So my recommendation would be to have a couple people take a cab or a shared ride van from the airport to wherever you are staying. Between that and the minivan, you should be able to get everyone and your luggage transported.