Ahoy, Kerri Ann!
Welcome to the Disney Parks Moms Panel! You've asked a great question, and I'm happy to provide you with some helpful information that I hope steers your friend or family member in the right direction.
My best advice is to actually go ahead and add the Guest in question to your reservation as long as you are before your final payment date. My first reason is that you want to lock in the current pricing because fares could go up as the cruise date draws closer and only a deposit has to be made at the time of booking. The second reason is that even though the stateroom you've booked may hold your additional family member or friend, there are certain lifeboat guidelines
Disney Cruise Line has to follow. If capacity is reached on your deck before you add this Guest, you will be out of luck, Kerri Ann. So, the good news is that you can cancel or change Guests before final payment is due and still receive a full refund. To figure out your final payment date, check your cruise reservation information because it can vary depending on number of nights your sailing, your particular itinerary or if it's a holiday sailing. After the final payment date, cancellation fees will apply, but you can change to another friend or family member up to 45 days prior to departure without any penalty or additional fees. A change fee of $50.00 is charged if within 45 days of the sail date.
Kerri Ann, I hope this all makes sense. If we can assist you further with your plans, please let us know.
Have a magical day!
Nancy