Help with outdoor early morning Group Shot

Brief scenario - I have been asked to take a group shot for some cyclists (probably about 50-60) before they set off on a ride this weekend, trouble is, they set off at 7:00 am (just on sunrise - first light is 6:30 am) but my biggest problem is that I've rarely used my 430EX flash so I'm looking for some help or guidance.

I'll be using a 40D and a 24-105 f4 L lens

The location of the shoot will be behind the local bike store, so will potentially be little ambient light, but I plan on doing some test shots on Friday and Saturday (the ride is Sunday) mornings at around the same time so I can get a feel for what I'm going to have to cope with, but current advice is to shoot Manual mode at say f5.6 and 1/100 at ISO800 and let the 430EX take care of the rest or would I be better going Av mode on a tripod (getting group that large to 'not move' for maybe 1 - 2 seconds may be a problem)

Any help or advice would be greatly appreciated (other than to learn how to use my flash properlyops

I would do some practice shot before. 50-60 folks is large group so you will be shooting from a distance. How is flash going to be mounted?

bobbyz wrote in post #5868668
I would do some practice shot before. 50-60 folks is large group so you will be shooting from a distance. How is flash going to be mounted?

The flash will be mounted on the camera and I estimate that I'll be about 10-12 metres away (the area behind the shop (car park) is sloping and I planned to have the group on the up side of the slope where I can hopefully catch a bit of the morning sky)

Hey guys, I really need some help with this

All I can say is use something like f8, f11 for aperture. Camera in manual mode. Your shutter speed will control ambient exposure and flash in eTTL mode. You will need to adjust flash through flash comp to see what it looks like. Having a flash meter is real handy for these situations. I would have used flash on something higher than camera shooting position. You should shoot from a bit higher position also. Maybe some step ladder if possible.

Flash output will drop at lower apertures and greater distance. So I would definitely do practice shots.