I should have also said use the slowest shutter speed you can get away with.

If the groups are relatively well behaved adults, tell them that you are using a slow shutter and ask them not to wiggle. 1/30 or even 1/15 may work. This technique allows more ambient light to reach the film and will not cause over exposure of the main subject, the auto mode of the Nikon strobe's control main exposure by limiting flash duration, that is much faster than the shutter, say maybe 1/1423rd for example. The strobe can see the ambient light too.