@Yolima4,
You can only add an additional guest for the time of the whole reservation.
Or they have to make 2 different reservations or you have to change the reservations by adding the extra guest for whole period and then decrease the calculated price manually for the real amount of nights.
Be sure you have good communications/agreements about this in the message system.
Other way round: add guest for whole period and refund afterwards the nights the additional guest was not there, then you are allways on the safe side.
Best regards,
Emiel