I don’t know how your group hikes are run – Are the organizers typically leading the way, with most of the other people not really paying much attention to the route?

GPS units and topo maps for everyone in the group may not be practical. However, the organizers should at least hand out simple photocopied trail maps with the day’s route highlighted and turns marked. When I’m out with even one other person and I’m doing the navigating, I like to think out loud (ie “Oh, here’s where the red trail splits off from the blue trail we’re on. We’ll follow it for a mile where we should hit the yellow trail.”)

The buddy system already mentioned is a good idea for a large group. And it sounds like you already make sure everyone is accounted for when the hike is over – well done!

A whistle might have been heard before the group got too far away.

Have a plan for what to do if you get separated – If you think you’re lost, stop and wait for us to find you. If you guessed at which fork to take and think you might have guessed wrong, backtrack to that point and wait there.

Just curious, but an account of what the lost person did during those two hours might be interesting and instructive.