Use a Field of View Calculator such as this one: http://www.scantips.com/lights/fieldofview.html
But be sure to do some test shots. I suspect you will want to find a larger work space and use a longer focal length to shoot groups of people.
The reason for this is that in order to get everyone in the image at such short distance you will need a fairly wide lens. According to the calculator, on an APS-C sensor camera such as your 7D, a 28mm lens will render just over 6 foot height (on the shorter axis, i.e. holding the camera in the "landscape" orientation). Using this wide a lens so close will inherently cause perspective exaggeration of the peoples' features and anamorphic distortions at the edges of the images that can be pretty ugly in portraits, in particular (unless you are trying for a humorous look).
You'll get nicer results working with a somewhat longer focal length in a less cramped space... such as 35, 40 or 50mm from 15 feet or more away. A zoom such as the EF-S 17-55mm or many others (EF-S 15-85mm, EF 24-105, EF 28-135) covers all these focal lengths from 28mm to 50mm and more. Alternatively, you could used fixed focal length lenses (i.e. "primes", instead of zooms). Sometimes prime lenses can be better corrected than zooms (less "barrel" or "pincushion" distortions, better sharpness across the whole image)... But modern, higher quality zooms now come pretty close to the quality of primes.
Experiment a bit and see what you think. You didn't mention what lenses you already have, maybe something covering these ranges.
Do some test shots and carefully check the size relationship of peoples' facial features (big noses, small ears) for perspective exaggeration... And watch out for positioning people too near the edge of the frame (especially on the longer axis), where there will be anamorphic distortions (the person's arm or leg closest to the edge looks larger than their other arm or leg, and it appears "stretched" out of shape).
Short to moderate telephotos (roughly 50mm to about 85mm on an APS-C format 7D) are the "traditional" portrait lenses for a reason... In photos of people this range of focal lengths will show the least distortion and render the most natural looking perspective. But you'll need more working space to use these.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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