@Patrick664
A host will specify a night rental fee and the number of guests allowed for that fee e.g. 4
They will also specify a maximum number for the apartment e.g. 6
The extra guest fee is the fee for guests 5 and 6 , should they be required
So, if you are 5 guests you pay the nightly fee and one extra guest fee
"My imagination is that, if I let someone else sneak into the apartment, such that more guests are living in the apartment than what's specified in the booking, then I'll have to pay the extra guest fee."
no..if you sneek in unpaid guests the reservation can be cancelled and you can be thrown out by the host...as it is a violation of the rental terms, ..although ..if you are lucky..you might be allowed to pay the extra fee..depends... if you allow in more than the maximum allowed....then you would be thrown out
(I am not saying you would do that..I just mean should the situation occur)