I think it depends on the restaurant. Some places could easily accommodate a large group with little notice, others, it would bury. I've been in places where, even with reservations made weeks in advance, a group of 20 turned out to almost overwhelm the place.
I've also had places where the guy was about to lock the door, just as we were approaching, who reopened because he was so glad to get two more customers -- but that was two people, so he could handle it himself.
I think it depends on the attitude, too. I can remember back in the old days working in retail, and you'd have dawdling customers who acted like you were peons and shouldn't mind that they were wandering around hours after closing. Them, you wanted to kill. But the frantic and grateful mom who was trying to do all her shopping on her one free night, you had no problem helping, however long it took.
So if you pick the right place and, as Cathy2 described, try to be particularly good customers, then it can work. However, if you can make reservations in advance, it's probably wise. And still try to be good customers.