We've had good experiences at Piola Pizza near Union Square (12th and Broadway):
Excellent pizza and salad, reasonable prices, cute decor, big enough for groups; not especially quiet, but we've generally been able to have conversations there with no problem.
Otto (near Washington Square - 5th Ave and 8th St) might be very good for this as well; though a little more expensive in terms of size vs. price, it's also group-friendly and has terrific food.
48 E 12th St, New York, NY 10003
How big a group? Any particular neighborhood? Willing to put up with a wait, or want to sit down right away?
Assuming a large group, I would say Patsy's would work well, multiple locations, solid pizza, good salads and not too long a wait usually. Other options are John's on Bleeker (longer wait, but better pizza) or Arturo's (would call for a res for a larger group.