Well, they usually state the price is for two people a unit and a car so you are paying for the car, but even so nobody complains about having to pay for an extra car and yet a car costs the campsite nothing.
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How do they know who the visitors are? If they just leave their car in the car park outside, or nearby, then walk onto the site as if they were staying there, I doubt anyone would challenge them. They can't possibly recognise everyone staying there. Even if the site wardens came over to your pitch and saw more people than were booked, they wouldn't know that the extra people weren't from another pitch on the site.
Visitor times 7am to 7pm? What if visitors turned up at 8pm? The site wardens would probably have gone off duty by then, and as long as you or your visitors didn't annoy anyone, I doubt whether anyone would even know you had outside visitors.