Before you rent anything go to Gemut.com (http://www.gemut.com/), read the downloadable brochure on renting a car in Europe and CALL Andy for advice. There are lots of "issues" with car rentals in Italy and you need to research that thoroughly, but talking to a person has been the best for getting a deal for us. If anyone can do it for your large group, Andy can, but as others have said, it will be pricey.
They advertise themselves as specialists on Germany, Austria and Switzerland, but they know about all of Europe. We have rented through them for the Benelux countries, France and an Alps driving trip through Germany, Austria, Italy, Switzerland and France.
Please be advised that at the moment their website is not playing nice, so have patience. This part about larger cars and Priceline in particular, should be required reading for you: http://www.gemut.com/bestors-blog.html.
Also be aware that in addition to the extra costs (like the mandatory CDW insurance) if you rent 2 cars, there will have to be 2 different drivers and the rental will have be charged to the credit cards of the 2 drivers or else you will be dinged with an extra fee for a 2nd driver. We found that out when we rented the car for France, charged the car to my CC, but my husband drove it. The fee was not charged until we turned the car in.
Update: I emailed Gemut yesterday about their website problems and promptly got this response -- We just upgraded our content management system and our webmaster is tying up loose ends. Sound familiar? It seems to happen with every upgrade or new website launch. However, the quick response to my email is indicative of the quality of the Bestors' service. And things seem to be much better today (3/13).