Its nearly impossible to get everyones eyes open. Bosaiya has it right when he sugests a loud, startling sound, causes most to get a bit bugeyed. It also helps to get your subjects looking to the side of the camera, and not at it. Have an assistant help direct them at an angle or divert their attention, while they watch your helper, take the shot. If you dont have help, then a long release cable(preferably electronic)to get you away from the camera while you distract them. Believe me, its tough, I used to do a lot of big groups when I worked for Olan Mills. I used to shoot anywhere from 400 to 450 sittings a week, and at least a third were groups of four and more. Good luck, and practice getting people to look where you want them, dont be afraid to give specific directions to individuals, and keep it light hearted and upbeat, that also helps.

Rick