@David7097 If the listing already has an Extra Guest Fee specified in the pricing settings, you can request payment of that. But if you want to change the booking to reflect an accurate guest count, you may have to temporarily change your occupancy maximum first.
The guests have put you in a bind if you're not confident that you can safely accommodate a group larger than your maximum. You have every right to turn them away, but that would usually mean forfeiting your payout.