Hi Kerry! Welcome to planDisney!
The more, the merrier, and it sounds like you are adding more loved ones to your upcoming trip! How exciting! I have visited with
extended family, as well as vacationed with lots of Disney-loving friends, and I can attest to just how much fun it is when we all come together at The Most Magical Place on Earth. You will have a blast!
It has been my experience that there are two options when adding a Guest to a
Disney Resort hotel stay: add them to the reservation for the entire stay, even knowing that they will arrive later in the reservation, or split your stay into two reservations, one with your original party and one with the extra person. I have always opted for the first option because this guarantees that I don't have to move to a different room during my stay, and if the extra Guest's plans change and they can join the trip sooner, they are already on the reservation! With the second option, you could be asked to change rooms since it is a separate reservation number.
Either way, I recommend
giving Disney a call at (407) 939-1936 and speaking with a Cast Member to either add the Guest for the entire stay or to split the reservation up. They can help you get it all sorted out before your arrival in just a few days!
I hope you have the BEST vacation, Kerry! Please let us know if you have any other questions.
Tabitha