Hello, Dorothy!
I get it. Sometimes life throws things at us that create conflicts when putting together a Disney Cruise Line vacation that is practically perfect in every way. Nevertheless, I sincerely hope things work out for your entire party to enjoy your cruise together.
As far as adding or removing a Guest on a stateroom reservation goes, you can usually choose to go either way. My recommendation would be to start with the whole party, with the understanding that a guest can be removed later, if needed. That way, you ensure that the assembly station for your reserved stateroom will have enough places for your party as other staterooms around you in the same assembly station book up. By booking everyone early, you also lock in today's cruise fare, protecting against possible increases in prevailing rates that would apply to additional Guests later on. On the other hand, removing a Guest from a reservation could be subject to cancellation fees for that Guest's fare according to the terms for the relevant stateroom category and reservation. Keeping those tradeoffs in mind while considering the most likely scenario for whether the third member of your party can make it on the cruise may help to decide whether it's better to book everyone at once now or possibly wait to add an extra passenger later.
An alternative approach might be to consider a modification of a person on your reservation, if necessary. That is, if a given Guest on your stateroom reservation can't go on the cruise after all but you have a friend or family member who could, you may be able to modify the members of your party on the reservation. Modifications like that could be subject to a change fee (currently $50, but subject to change) if the modification is made during the period when cancellation fees would apply to the reservation.
My understanding is that it is relatively common for Guests to add, remove, or modify members of their party on a Disney Cruise Line reservation. Modifications and cancellations can be made within My Reservations or by calling the Disney Cruise Line directly. However, if you should book your Disney cruise through an authorized Disney vacation planner or through the Disney Vacation Club, only that third party would be able to contact Disney Cruise Line to make those changes on your behalf.
I hope the booking goes well for you, Dorothy. Here at planDisney, we are fellow Guests like you. That means you may ask us for any advice you may need as you continue your cruise booking and planning and we'll be able to support you from the perspective of someone who has "been there, done that" before. Please don't hesitate to visit us again with any more questions you may have.
Have a magical cruise, Dorothy!
~Tony