I think you need 1) a SINGLE, very powerful flash unit with 2) no umbrella or softbox of ANY kind, because it's not gonna do a damned bit of good on a 60-person shot, and 3) a very HIGH placement of the flash head, to mimic the sun. There is only ONE sun. (on this planet,at least.) And it does a great job of lighting up huge stadiums, entire mountains,etc.,etc. So...like the best of the old-time banquet photographers, you do not need,or want, any diffusion over the light. NONE! For multiple reasons.
You need to get the light up HIGH, so the shadows fall more or less almost straight down, as if the sun were at 11:30 or 1:00. This looks very natural, and the shadow dropping down at a very acute angle means that the shadow from a person in front of another person will basically, NOT fall on the face of the person behind, nor to the sides. THIS IS CRITICAL!!!!!
I would shoot this with about a 50 degree beam-spread 11.5 inch diameter polished silver interior reflector and a Speedotron 2401B pack firing at 2,400 Watt-seconds through one flash head. If you JACK THE ISO UP to 640 ISO, you could get by with a lot less power. I would really,really want to shoot this with a BIG flash. Failing that, 3, or 4 flashes literally PC-connected in series and triggered all at once is an alternative--but you're probably going to need elevated ISO level to make it all come together. Shooting a 60-person group after sunset is the kind of scenario where having the RIGHT equipment makes it relatively simple to expect decent results.
The flash cannot be "too close", or the fall-off will be terrific...it needs to be placed BACK a ways, to even out the light's fall-off...so,once again, the need for massive flash power...
