I think a softbox and the umbrella will be helpful. I would put a flash off to either side of the group at a 45 degree angle just at head height and then maybe one more in front up higher and angled down slightly. The last one up high from the back side with low power just to highlight the edges (hair light). I like the idea of compressing the BG but you will have to be probably 50 yards away with 500 and might be tough to not include your flash gear in the shot.