Expanding on the idea of using a spreadsheet to calc the coverage of a particular lens ahead of time....
If subject is 5' tall and 10' away, and you're shooting with a 100 mm lens on 35mm film:
60"/120" gives a height / distance ratio of .5
So, the lens used will also give a height /distance ratio of .5. In the calcs below I'll stick in a 100mm lens:
image height / focal length = .5
image height / 100mm = .5
50mm / 100mm = .5
Image height of 50mm is too high, since you've only got 24mm film height. So you can change the distance of the group, change the focal length of the lens, or both.
I actually made up an Excel spreadsheet to take the drudgery out of it, and make the calc very fast, so I know ahead of time what the image size will be, and can then concentrate more on composition instead of fretting quite so much about the technical details.
By doing a spreadsheet, assuming you're going to split the group into two rows, tallest person is 6', use a 90mm lens, 4x5 film, and a distance from subject-to-camera of 20', I get an image *height* of 27mm. Assuming the subject width to be about 19.5', I get an image width of about 88mm.
So, will your flash cover 20' distance? Well, more like 22' distance, so the back row is lit, too.
Also bear in mind photography basics, ie, the lens may have unflattering aberrations at the edges, so you may want to leave some space around the subjects.