Go to www.slowtrav.com and find the article about planning for large groups. Also, looks at www.summerinitaly.com for rentals both in Tuscany and the Amalfi Coast. Your task is almost impossible without help. You will likely need 3 vehicles and they may be manual stick--is that OK?

Well, you may all think you want to stick together now. But that will change once you're on the trip. With such a large group, I think it's important for people to have private down time. Older people get tired, the teenagers want to go out late -- and then sleep in in the morning. Some people love cathedrals and museums, others want to go shopping. And so on.

Fid an experienced travel agent ASAP and let him or her help you. June is quite short notice when dealing with so many people, 7+ hotel rooms or large villas. You might not all be able to stay in the same hotel. Or, if you can, it'll be some generic chain hotel far away from the sights.

Good luck!

Well 13 people doing everything together, every day for 12 days is going to be very cumbersome!

The cost of a total trip is impossible to ballpark estimate with only the information you've given so far... Depends what you can find for plane tickets (largest single expense), what kinds of hotels/apartments you choose to stay at, kinds of restaurants where you eat, city tours or self-guide, car rental or public transportation, day trips, etc. etc.