Group eating in Rome

Early depends on the location. You will see people in the tourist traps along major walkways (by Pantheon, etc.) at all hours. 8:00PM is about as early as a Roman shows up, and 9:00 is peak time (like 7:00 on Saturday in my home city). Reservations ANYWHERE ensure some assurance and if you know where you want to eat, even if it is 7:00PM, make a res. Otherwise show up before 8:00PM.

Six isn't that large of a group, and even a smaller trattoria should be able to accommodate you. Agree that go early, 7PM and a reservation wouldn't hurt either.

Go to "Hostaria da Settimio" at Monteverde and you'll eat like a local.
No written manu, no English spoken, no tourists.
Make reservations.
Romans show up after 9pm, but if you go earlier the place will be emptier and it will be easier to order in a foreign language (unless you are fluent in "Romanesco").
It's a short tram ride from Trastevere (tram no. 8), or just take a taxi. It's near the San Camillo Medical Center.
http://www.yelp.com/biz/hostaria-da-settimio-roma.

American tourists are a boon to European restaurants. We like to eat earlier, so they can have a second seating of locals. It works out for everyone.

Thanks all for the replies. To be honest, if I can make my in-laws hold out until 8pm for dinner it will be a miracle anyway, so it sounds like the timing will work out well. Roberto, that place sounds great- if the missus and I can sneak away for an evening on our own we will go there. The two of us have fun getting by with our remedial Italian pleasantries and hand gestures but I won't be able to translate for the rest of the group, who are not as adventurous (I also doubt they'll want to stray that far from the center).