I realize it's past your family's time of travel but the info I'd offer for people looking at this in the future is as follows. Almost everywhere is going to charge a price per person unless you know a driver from the past who has easy access to a bus that they don't have to rent. It's just the way it is there. The cheapest transport you're going to generally find for that particular commute if it is not included in your booking at the hotel is to just go through JUTA itself if the resort doesn't have a better option. There is a desk at the airport. With that large of a group it probably won't even be a shared ride with others like it is for most. I doubt you'll find it for much less than $10/person one way though. Here is a link to get a quote from the operator at the airport: JUTA airport transfers at MBJ: https://www.mbjairport.com/taxi-cabs

In all honesty $160 US roundtrip for 9 people is not a bad price. That's a little less than $9/person one way.