@Terra112 Be very sure that you are willing to add extra guests -- the wear and tear can be considerable. Also, no matter what you do, there are going to be those groups who want to cram as many people as they can into a house so that their portion of shared costs go down.
IMO, the charge for the extra guests would be a ratio of the nightly fee. For example, if you are going to charge x dollars per night for 6 guests, and want to have a figure for 8 or more guests, divide your nightly rate by 6 -- the resulting number would be the nightly charge for guests 7 and 8 (or 9 and 10).
$300/6 = 50, charge $50 or $60 per night per extra guest.