You don’t say if you are working inside or outdoors, but as no one else has mentioned it, here’s another method.
Use everything in manual mode and take a light reading, choose an aperture of at least f8 (f11 would be better). This will probably give you a very slow shutter speed. So you’ll have to use a tripod and tell everyone to stay still. Now set the flash to the same same f stop and use it as a fill flash.
Problems using this method are that everyone will have to stay still, no problem with a military group, but not an option with a bunch of toddlers. If you are shooting inside, the ambient light will give a colour cast, so you’ll need an appropriate gel over your flash so you don’t get mixed lighting.
All in all, the solutions that other people have mentioned are probably better, but as I said, no one has mentioned this yet.