Pick your spot early, and take a little time checking framing. Bring at least one friend to stand here, stand there, see how tall people really are, how wide your field of view really is, where the trees and pipes and radio towers are that are gonna grow out of people's heads. Take some tests to verify actual framing, and then plan to give yourself a little more room around the edge of the frame than that -- not too much, you're gonna have enough trouble maintaining detail in people's faces without cropping like crazy!
It'll take a while to get everyone pulled together for the shot. Have the people who show up first stand in the three rows, and out at the sides, so you can take some test shots as people show up. Then chimp like crazy, zoom in on the LCD and see what's in focus and what's not, and how it all looks.