When I photograph large groups, I cluster my lights a few feet directly above the camera. When I'm using six monolights, each outfitted with a large umbrella, the result is a solid cluster of lighting that is about six feet high and 12 feet wide. It's like a giant softbox. I never get the shadow of a person on the people behind him/her.
When I started doing groups, I tried the two clusters of lights located right and left of the camera and got the very same horrid shadows you're seeing. Simply relocating the lights as I suggested above and using the largest umbrellas (reflective umbrellas, that is, NOT shoot-through umbrellas because they waste a lot of light) that your lights can fill on each light will solve your problem.