I do this exact thing every year, and here is how I do it. I take three powerful A/C powered studio strobes and I fire them into three large umbrellas, all on 13 foot stands raised up nice and high and aimed toward the group. All three are in a straight line above and behind the camera, one is centered on the group the other two are positioned at the outer edges of the group. The left one is aimed at the people on the extreme right, the right one is aimed at the people on the extreme left. I feather them until I get a meter reading of approximately the same all the way across. If the back row is underexposed by a stop due to fall off, don't worry as it can be adjusted later in Ps. I simply get the majority of the group exposed properly then back in the work room in ACR I get the front rows properly exposed and save that file. Then I raise the section that was underexposed up to where it is properly exposed and save the file again. In Ps I combine the two images and erase the overexposure from the overexposed section leaving the formerly underexposed section (which is now properly exposed) alone. I now have one perfectly exposed image.
Shoot in Raw. Shoot a gray card first. Use a tripod, and use a shutter speed that allows the available light to expose the background. I use a cable release to prevent camera possible shake. Take accurate meter readings. Shoot plenty of images so you will have a number of files to use to make one good one where everyone will have their eyes open. WATCH FOR EYEGLASS GLARE. Right at the end of the shoot ask everyone who wears eyeglasses to remove them and shoot several images. That way you will have "good" eyes to use to make one image with no glare.
Benji
