We are a group of 4. RS says it is 40euros by taxi to downtown. We are staying at Casa Di Accoglienza on Viale Vaticano near the Vatican. Would that be the same fare? It sounds really easy instead of trains and buses!
Roman city bylaws say it's 40 Euro for a cab ride inside the Aurelian Wall....up to 4 passengers and luggage included. Go out to the taxi rank and look for the lineup of white official cabs....don't go with anyone who approaches you in the terminal. Unfortunately, Viale Vaticano is outside the walls, so you will pay more. But check with the TI in the airport to see what you should pay, then ask the first driver in line, telling him what the TI said would be an average fare. If he/she baulks at that, go to the next cab in line. Make sure they run the meter. You will pay more for a metered run on weekends and late at night.
Also. the amount of luggage will be critical. Four people and four pieces of carryon luggage will fill most cabs. You may have to look for a bigger cab or a small van if you have more than one piece of luggage per person.