Is this a touring 'van or a static ?
Its sounds as if the seventh person is only a baby, and you can always squeeze a baby in somewhere !
It doesn't really matter if the people who are hiring it "find out" or not.
But bear in mind...if this is a touring van, 7 is quite a lot to occupy the 'van.........you will be very hugger-mugger. I hope you are all very good friends !