Depending on the number of people taking part in the event, you may consider a marshal to be part of the event to act as leader, sweeper, backmarker or support vehicle:
Leader: Knows the route well and sets a pace to suit the majority
Sweeper: Continually travelling up and down the route to make sure the pack is together
Backmarker: Stays at the back to provide assistance and ensure nobody is left behind
Support Vehicle: Carrying First Aid Kit, refreshments, spare equipment
Larger groups will naturally spread out over the course of the event, so it is really useful to have these roles in place to ensure all participants are accounted for during the event. Road Marshals may also be considered if your event uses road crossings. It is helpful if larger groups cross the road in smaller numbers. Ensure Marshals are easily identifiable.