You can call the dining reservation line to see if they can add more people to your reservation. If the new group size you need is fully booked the CM won't be able to do it. You would then have to start checking online to see if you can find a reservation for your new group size or a second reservation for the additional people at a time as close to your current reservation as possible. If you find an additional reservation for the extra people you should make it in the name and phone number of someone who will be using that reservation because the system will usually ask if you want to cancel your current reservation because the time is too close to the new reservation.
You cannot make any changes to the size of your party online which is what it sounds like you are doing.
How many people are you trying to add, at what restaurant and what time is your dinner? All this will make a difference whether you'll be able to find a new reservation. This close to the dinner date you may not be able to find anything so you should have a Plan B in mind. As a last resort, if you can't find anything your extra guests could try walking up, but there's no guarantee that they will be seated so they should be prepared to eat at quick service or at a different less busy restaurant.
