I dealt with this just a few weeks ago, I needed to add 2 extra people for all our dinner reservations. And unfortunately for ADRs, you cannot change the amount of people within your party on the site, you'll have to call and speak to someone at Disney. When on the phone, I asked the woman if perhaps I had just overlooked something on the site, but she confirmed that changing the party number cannot be done on MDE as of yet.
Also, depending on the restaurant's availability, they might not be to accomodate you at the exact same time. One of our reservations was for the California Grill, and in order to accomodate the number of people i now needed, she had to bump our reservation out half an hour from our original time. If you have reservations at a super popular restaurant, you very well could be in the same boat. Good luck.