Welcome to planDisney, Doremi!
I am famously indecisive. I like to plan our trips pretty far in advance, but often the members of our traveling party or our 
resort hotel change a couple of months before we leave. 
Flexibility is one reason that my family likes to book a Walt Disney World Resort vacation package as opposed to purchasing our 
theme park tickets separate from our accommodations. 
You see, when you book a vacation package, your entire trip is refundable and completely modifiable up to 30-days before you depart. Please note that payment terms and conditions are subject to change. 
Since it is much easier to remove people from a reservation than it is to add them, I recommend that you go ahead and plan for your entire party. Simply pay the deposit for now and make a final decision when it gets closer to the time. Just be sure that you keep an eye on the date so that you do not incur any fees. 
I hope that this answered your question, Doremi. 
See you real soon!
Sarah B.