Agree with ses1230. As long as at least one person shows up for the reservation you will not be charged for the missing couple.
On the other hand, showing up with an extra person can be a problem. It's not a matter of squeezing in one more person at the table, but fire code regulations. While CMs will do their best to accommodate extra people, there's the outside chance that the restaurant is at capacity and won't be able to seat any extras. Also, we were at a DW restaurant that was running behind schedule so they turned away walk ups, latecomers and anyone not included in a reservation.
If you are expecting additional people you can check online multiple times a day for a reservation for the correct party size. You could also look for a reservation at the same time for the additional diners and when you check in ask if you can be seated together or near each other. Again, there's no guarantee, but at least everyone will be assured of a seat in the restaurant. You can also subscribe to the cancellation thread for your month that is on this Dining Reservations board.