Have people in shot with manual flashes, turn off headlamps, open shutter, one, two, three, flash, close shutter, headlamps on.
Careful positioning of people and flashes usually adds depth to the picture, randomly placed slave flashes triggered by the manual ones can also add to the effect.
I used to use camera on tripod with cable release to avoid judder, can you fit a cable release on a digital camera?
Always remember, nature is in charge, get it wrong and it is you who suffers!.