@Katie318 honestly, hosts shouldn't *have* to say that guests should not bring extra people to stay who aren't on the reservation.
Do restaurants have to say "Please don't take extra food that you did not order"?
Do grocery stores have to say "Please don't take extra items that you did not pay for"?
Do hair salons have to say "Please don't expect us to cut and style the hair of friends who come with you"?
You should at least put in your house rules "Unregistered guests are not allowed on the property at any time." That should cover any eventuality... although enforcement is up to you.
And tell the guests that unregistered "visitors" need to either be put on the reservation (with a potential increase in cost) or must leave the property immediately. Failure to do either would result in the cancellation of the entire reservation, such that all guests would have to leave.
The pricing of extra guests is entirely up to you.