check out discussion here: http://www.chowhound.com/topics/346612
Riva destra is a gorgeous place with delicious food, but it is somewhat off the beaten path and not particularly traditionally Italian.
The chef (English), Allan, at Cantina del Vecchio has made me a great menu for our rehearsal dinner. http://www.cantinadelvecchio.it/default.asp?lang=en
Casina Valadier has very nice private rooms with gorgeous views for your size group. http://www.cantinadelvecchio.it/default.asp?lang=en at the top of the Spanish Steps
Hotel Gladiatori is right near the College, and has a nice terrace - wasn't big enough for us, but I'd have considered it otherwise - great view! http://www.hotelgladiatori.it/it/roof_garden
http://www.hotelcapodafrica.com/capoenglish.html also has terrace, is very close to College and does catering for private events.
Ditto hotel Forum, just not quite as close to the College as Gladiatori http://www.hotelforum.com/
Good luck!