If you are searching to be a guest, the price you see will be the price per night and will already take into account the host's additional person fees. For example, if a rental is 100 money units (pound, dollar, euro..), but has an additional person fee of 20 MU, then when you search and put in "two adults" for your search, then the price you will see will be 120MU. If you searched for the same unit as one person, then the price you would see would be 100.
If you are a host setting up a listing, then you put in a base rate, and then your additional person fees, as Gerry and Rashid said.