Its actually very flexible. You can change or cancel the fp+ for anyone in your party, even have the people going to different parks. You will always be asked, when you say you want to change a FP+, which people you want to make the change for. I find this easier and have more peace of mind doing this on the computer vs the app. It seems to ask the "Are you sure" type questions more. I could be wrong. I currently have a trip planned for a group of 9 with three resort rooms and made the FP+ reservations in three different scheduling sessions as people got their tickets. I was able to use the feature to make duplicate FP+ reservations for friends and family and got almost everything we wanted to match. I did have to move one thing around to get us all together. I completed the process at about 45 days out and this is for Easter week. I also sometimes make FP+ reservations for the whole party to a ride that I'm too scared to ride. Then I go back and change just one or two of them to Buzz Lightyear or another non-rollercoaster. Long story short, you can cancel or change just the other family's FP+ without messing yours up.
