Max persons on a French Pitch

Looking to go to Atlantic coast, France in May. There's 7 of us wanting to go and we'd like to take my 4 berth Unicorn Bareclona plus a 5 berth tent. Most sites seem to only allow 6 persons in total on their booking forms. I've heard loads of stories of French sites having 15 people on one pitch in vans, campers and tents....

Have found various CC club sites in the brochure over 120 sq m which would be big enough

Any advice on how to achieve what we want to achieve ?