Practice the Hunter Game.
It can be done in an arena or out in the field with more experienced riders. Put all of the riders in a line, they can practice transitions within gates by rating their own horse to keep the same pace and safe spacing with all the other horses. The game starts with the rider in the back pulling out of line and trotting to the front of the line, pulling back in line (watching spacing), then asking for the downward transition to walk. Then the person now at the end of the line does the same. When the whole line has gone, it's time to reverse or change the transitions.
Depending on the level of the rider, you might have the last horse canter to the head of the line or go from sitting trot to extended trot to get to the front of the line. Or you could have the whole line trot and have the person in front pull to one side, transition to walk, let all the horses go by then pick up trot again. The game can go on and on in a variety of paces. It is a fun game out in the open as the leader gets to pick the path, so the line can wander all over the place.
In addition to practicing transitions, this game also develops the control the rider has over the horse, by getting the horse to work independently, while still maintaining the comfort of being with the herd.