If I remember correctly, you just scan your magic band, no username or password needed. You will then be able to select a new FPP for everyone in your party (you do not need their MBs.) Even if there are no options that interest you, you can schedule something anyway then go back through your MDE to watch for desired attractions/times to switch to. There are always CMs available to help you if needed.
We just got back from our trip and I was surprised to learn that we couldn’t book a 4th, 5th, etc via the app. We had to go directly to a kiosk. I just scanned my magic band and I was able to choose whether I wanted the next FP for just myself or everyone in my party. Once it was saved at the kiosk, I could view/edit on my phone/app.
Something else I wasn’t aware of - once you use your first 3 FPs, you have to make your 4th, 5th, etc. one at a time - meaning I had to make the 4th FP and couldn’t make the 5th until I used the 4th
That might have been obvious but I just overlooked that stipulation prior to our trip.
@catdrj and @keithloveswaffles got it right. Tap your band, then look what’s available. Cast Members are Epcot today were suggesting using your fingernail. However, if you have trouble, there are usually at least two Cast Members hovering nearby to help anyone through the process.
In Magic Kingdom, with so many attractions, it is more likely that you’ll find something that you want. In Epcot today, just after 2:00 PM I check what was available for the evening and found nothing useful.