I don't see why you would need a diffuser if you going to bounce the flash. Small aperture will be needed indeed. You will have to line up your group in several rows, so you will need a depth of field of at least 3 feet. If you are shooting from 10 feet away at f/5.6 you get D.o.F. of about 4 feet. To be safe I would go at least to f/8 and focus on somebody 1/3 of the way into the group.