Do you have a package booked, if so then everyone has to have the same components and you will have to pay for them the entire trip.
If not;
If the extra adult is joining you at the beginning of the trip you can let the CM at the front desk know when he/she leaves and they won't charge you for the last 3 nights.
If the extra adult is joining you during the trip just wait and add them after you check in for the appropriate number of nights.
My sister always joins us for portions of our trip so I have done both of the above scenarios without any issues.