Welcome to planDisney, Leanna!
You can add another member to your party after you book your initial reservation; however, I can tell you from personal experience that it is much easier to remove a member of your party from your traveling plans than it is to add one.
You see, once you book your reservations, you can make your
Disney Park Pass system reservations. Then, 60-days before your check-in date you can make your
advanced dining reservations.
If you add a member to your traveling party after you originally made your plans, you run the risk of not being able to add them to your Disney Park Pass system reservations if the park has
booked up, or to your advanced dining reservations if the restaurant has no availability.
Leanna, if you book a
Walt Disney World Resort vacation package, you can do so for as little as $200 down per hotel room, regardless of the number of people in your traveling party. You can then add or remove party members with no penalty up until 30-days before your check-in date.
I hope that this gave you a bit of insight, Leanna.
Happy planning!
Sarah B.