This is going to be easier to do as a reservation modification where you put in the total cost - ABB doesn't have a mechanism to count different amounts of guests on individual nights.
Go to reservation modification. Make note of the current charge @6 total guests.
It looks like you have an extra charge of $59/night after 2 guests, so 6 * $59= $354 for the extra 6 people for one night.
Add the $354 to the original 6 person total & modify the number of guests to 12. Be sure to make a note that 12 guests are authorized for one night only.
Let your guest know that they'll need to accept the new reservation before they arrive & you should be all set.