when i went on the Keys to the Kingdom tour they talked about something similiar. a family or a couple usually get picked to be special guests that ride out on a float just before the afternoon parade. anyway, she explained how they pick and choose them, so maybe it's the same? she said that sometimes it's people wearing the bride and groom mickey hats, or it's families who look like they're so obsessed with Disney lol. they said they sit at the front of the park when it opens and they try to pick the people, trying to see who is the "happiest" person in the park.