The intensity of a light source, even with a softbox, begins to fall off at a certain point on the edges. Another way of saying this is that the center of the light will have a hotter output than the edges. A technique that is used by professional DP's is called "shotgunning" and is quite simple. Place the key light off to one side of the subjects and then adjust it so that it is actually pointing in front of them more than right at them. If you can find the spot where the falloff is hitting the closest subject with the same intensity that the center of the light is hitting the furthest subject, you will have nice even lighting. Pretty simple, but it can make a huge difference in the look of your shots.
