There are many camping forums on the Web that organize ralleys and campouts. Some that I belong to are RV Itch, Popup Explorer, Canvass Campers, Mid Atlantic Campers and Pop up Portal. Most members of these forums are Good Sam members and we organize get togethers all over the country, both Canada and the US. Some are highly organized but for the most part it is posting on the forum a message to the effect that I will be camping at such and such a place on whatever date, anybody want to join me?
Through Pop Up Explorer we did Gathering at the Glen (Watkins Glen State Park, Watkins glen, NY) for 5 years. One year we had 80+ families. Last year we joined Canvass Campers for a weekend at Gettysburgh. Several friends got together this year for race weekend at Watkins Glen, just by posting that they were going to be there and asking if anyone wanted to meet up. This year we will be attending a our 6th rally at Ives Run (ACoE campground at Tioga, PA)with the MFCC (Magical Flying Camper Club) a sub group of RV Itch forum.
There are many ways to accomplish what you are suggesting without formally having to make it a Good Sam organized event (not wanting to take anything away from Good Sam).