Once you get off the ride, there will be a preview area for all the pictures. Underneath the screens will be the Mickey dots that look like the ones used to get through the FastPass line, purchase things with your Magic Band, etc. When yours comes up, just hold your MB up to it, and it'll link to your account.
When we went as a party of 8 just a few years ago, we usually just had one person (if we went as a group) tap for the picture. It should only show up once (or twice, if you tapped too). Hope that helps!