If you want it in a park, I would suggest Alfredo's. They have a semi-private dining room that will seat up to about 40 people. If you call Richard, the area manager, he can tell you when it would be available. There are no extra charges for using it and they can easily set up a table for 12.
For not in a park, Spoodles (Boardwalk) has a small section which will seat about 20. Again contact the restaurant manager directly to see about availability. They have no problem, in that area, setting up a table for 12.
There may be others. However, you would need to make your PS reservations for right at the time the restaurany opens for dinner. That way you could be fairly sure of keeping the group together.
A last item, which might be a bit on the expensive side. Victoria and Albert has a special room which is used for groups of 8 to 12 or certain VIPs/Celebrities who have a need for privacy.