When I planned for a group of 12, disney IT recommended trying for the whole group of 12 first, then editing the specific FP if 3-4 people wanted something else. I.e. Picked 7dwarfs, Peter Pan and Splash Mointain for all of us, but 3 don't want to ride Splash. Book everyone. Then edit just those 3 people to a different ride. In my case I was able to get all rides for us all together as a group of 12. However, many people report having challenges getting what they want when they want it and have luck looking instead for two groups of 4 vs the whole group together. (I looked for the very in demand 7dwarfs at midnight of our 60 day window, but did all the rest midafternoon the next day and re-arranged them several times in the weeks after with good availability).
The link Mesaboy gave you was my "bible" for choosing and booking FPs and it worked out great!