We arrange our cars through the broker Autoeurope [for which we have a toll free number]; get zero- deductible collision, which covers any damage to the body of the vehicle. They will recommend the best agency based on your particular situation; possibly where the car is being returned. Read this about driving in Italy and restrictions involved [ZTLs]; avoid leaving anything of value in a car when parked:

http://driventoit.blogspot.com/

Last year we rented a 7 passenger van (automatic) through Auto Europe. Picked up in Florence and dropped off in Siena. Have rented through Auto Europe a number of times throughout Europe and they not only had the best rate, but were great to work with when we had an accident in France a couple of years ago.

I should have mentioned that we had the option of renting a 9-person van, but opted to pack lighter and take a smaller vehicle.

Sandy

8 occupants plus luggage will be challenging, even in a 9 passenger van.

A quick look reveals the 9 seat vans show 2 big + 2 small bags. Any van over 7 seats is a manual transmission.

Using 2 vehicles an option?

To get a sense of scale of driving a 9 passenger van in Italy, you may want to drive this ( https://www.uhaul.com/Truck-Rentals/20ft-Moving-Truck/ ) around as your daily driver for a week, before your trip.

Where will you be stopping between Florence/Siena and Sorrento?
Parking a van that size will be an issue and driving in/out if any small towns will as well.

It gets increasingly difficult to travel with so many people.
Many would find it easier to go in 2 vehicles rather than one huge van.

1--they are much cheaper.

2--it's hard to get so many people wanting to go to the same place.

3--your group could split up and see the sights they want to see separately.

4--everyone will not get on each others' nerves nearly as much.

Another suggestion--everyone carries one 21" carry on bag. Groups' stuff will get overwhelming if you carry more.

Just to emphasize some important points.

Follow David’s advice.

1 rental van costs more than 3 economy rental cars.