Hi Christina,
If the check-in date is passed, there is no possibility to change the number of guests through the alteration tool. In that case, you best ask for the extra amount through the Resolution Center.
If you do not agree with the change, you may inform the guest that only the approved amount of persons is allowed.
If the check-in date is in the future, you can send an alteration request which the guest then can accept. Upon acceptance of the request, the guest will be charged according to your price settings. If the guest does not accept the request, the former mentioned condition also applies.
Does this help?