Hey there, hi there, Karen! Welcome, and thank you for your question.
You can purchase
Walt Disney World theme park tickets at any time, even if you've already purchased
admission tickets for several members of your family. If someone else decides to join later, you can purchase their ticket at that time.
But, here's where it gets tricky. Even if you have a theme park ticket, you must also book a
theme park reservation for the days you want to go into the parks. Before you purchase tickets for your family, you should consult the
theme park reservation availability calendar to make sure the parks you want to go to still have reservations available. Right now, all of the parks are available around the time of your vacation - which is coming up so quickly!
If you decide later to add someone to your party, in order for them to go into the same parks you are going into, they will have to be able to secure a reservation for those parks. You can view the calendar prior to purchasing the tickets to make sure there's availability, but I can't recommend enough that you should try to firm up your plans as soon as you can. I'd hate for someone to be disappointed!
I hope this information helps with your planning, Karen!
Anna