Who do you recommend for a transfer service from Rome city center to the port and from the port to an airport hotel for a group of 12?
There are so many good limo/taxi/shuttle services. See:
https://www.tripadvisor.com/Attractions-g187791-Activities-c59-t182-Rome_Lazio.html
You could also go onto Port of Civitaveccia and they have websites for local shuttles that pickup in Rome.
We've used a private car service based in Rome a number of times and have only good things to say about them. The company is Rome Cabs, but it is a transfer service company despite the name. The web site is romeCabs.com. Check out the web site and see what you think. For a group your size just email them detailing your needs. They usually respond within twenty four hours.
If Rome Cabs is booked, they have a sister company, which we used. I believe same prices.
http://www.rome-chauffeur.com