Lets see, the OP states that there was only 2 of them at the site. That site most likely entitles them to have 6 or 8 people camping, & using the facilities.
They have a couple of guests for a few hours. Now we have 4, still under the allowable number.
Where do these KOA, & other, CGs get off charging extra for guests when the total is still under the allowable number of people per site?
A situation like this caused me, well, the DW, to boycott KOA for 7 years. We will never return to the CG that hit us for guests who were only coming in for dinner & we were under our allowable number for the site.