We have had contact for Napels with 4 companies.
Prices from 380 to 500 euro.
The companpie most interested for us (4 people)was Italy car services.
we have a price of 380 euro for the whole day, to visit Sorrento and the Amalfi Coast (Positano and Ravello stops). I don't know if they have cars for 8 persons. We haven't book yet, so I will ask them if you want so. You can visti there website www.italycarservice.it
For Rome we have decided to do it on our own by train.