You make a FP+ reservation for yourself, or one person in your party. Then you go back and instead of selecting "Make a New FP+", you select the Copy option. It will then allow you to copy your FP+ reservation to the other members in your
MDE account. If the exact time isn't available, it will copy a similar time frame that overlaps. If the attraction isn't available at all, it will assign them a completely different attraction. So the attraction has to be available still.