Formal Portrait Lighting

I am trying to find the best combination of light, diffusion and portability for when I take large group formal portraits. I have two 580EX's and an ST-E2 which will fire them remotely when positioned to my left and right but I feel the shadows are too harsh. I haven't tried umbrellas yet which I think will help a lot. I've put a variety of diffusers on which help but I feel I need to take this to a new level. I need suggestions to pull this off without too much gear and setup. Any recommendations?

do you have more info?
indoors, outdoors? how big is a large group to you?

I have two lightweight light 7 ft stands with two umbrella brackets on them. Two spare 580's to go on them. two 32" convertible umbrellas. It all fits in one long, but little bag and is super light, and super portable. I've lit groups of 30 with it.

http://www.strobist.bl​ogspot.com
Try this website. They specialize on this type of lighting technique. They have lots of information........