Push off with left or right leg
I taught my daughter to use clipless pedals by putting her bike on a wind trainer while she watched television. I got her pedals that are flat on one side with SPD's on the other side.

A random intervals I'd call "left foot down", or "right foot down", trying to time my call for the most inconvenient moment. And as I mentioned in comments, I never did tell her that everyone eventually falls.

When she was comfortable with those exercises, she started riding on the road with her new pedals. Sometimes she would ride with the flat pedals, but she quickly came to like the feeling of confidence that comes from knowing one's feet wont slip off the pedals.

After four years she still hasn't (yet) fallen because of the pedals.

To answer your question - it doesn't really matter which leg they usually push off with. 

But it does matter that they learn to be able to start and stop on either leg. In my observation falls come when the rider "has the pedals wrong" when they make an emergency stop. They need to be confidently able to unclip and stop with either foot.

Edit - re the additional points.

To restate my previous paragraph in a different way: it matters for her safety that she be able to start and stop on either foot. Not all stops are preplanned. It's the sudden stops that lead to falls, where the rider gets themselves tangled up trying to get the wrong foot out of their pedal.

For starting, it's a great help to be able to start from either foot also. It's normal to have a favoured foot, and it doesn't matter which one it is. 

And if most of the bunch uses one leg, would it be better to do the same so that you all lean toward the same direction when you stop at lights etc? No, IMHO you are over-thinking it at this point. I suspect you are thinking that because the bikes are leaning when they are stationary, that everyone will wobble or turn in that direction as they start off. If that is what happens, then I suggest that they are not ready to ride on the road. In practice, I just don't see experienced riders (including my own daughter) wobble that much. And the amount they do wobble seems random: members of the bunch push off at slightly different times, and make their first pedal stroke with different levels of effort. And when they stop, the bunch will generally stop in a random arrangement, depending on how hard they were concentrating, who they were taking to, what gear change they were trying to make, etc, etc.

So the conclusion hasn't changed: no it doesn't matter which leg, but she should be able to use either.