I would try shooting that in open shade. That is a ton of people to try and light with flashes. I did it a while back, but that was using two speedlights with large umbrellas in a white walled room. I still had a tough time with it. If you are looking for something to assist in lighting in the shade, you could get a huge reflector. I just purchased this one for a family shoot outside in a couple weeks. I actually just used it for a white background on a headshot yesterday...works pretty well! If weather doesn't cooperate, I would see about rescheduling for another time then trying to find something indoors.