Any time you are riding out with other horses you can do a little practice. Although the other riders may not think you are so friendly! Horse before social time - sigh.
Going in the same direction as the group, drift as far from them as your horse will tolerate without noise and/or other fuss. You are still getting where you are going, on a parallel track. Same when parked with the group in the middle of the ring waiting on another rider. Don’t go further apart than he is comfortable, even if it is 6 feet. While he is at the maximum separation that he can handle, reward, reward, reward - voice and petting. Horses do work for that, although sometimes it is not so obvious - especially if it is consistent. Give him something to do - laterals, etc. - to help maintain quiet and distance.
If a trainer says something about staying together, explain you are training. And stand your ground so you can actually have a better-behaved horse. My biggest problem with separation is not the horse - he’s come so far and does fine - it’s the trainer who keeps trying to push everyone together, although for good reason, I do understand.
Any group lesson is an opportunity - don’t waste it as this takes time and no progress is made when it isn’t being worked. A day’s 2-hour schooling offers a great opportunity to open that separation distance quite a lot - if one is consistent, consistent, consistent. He may get a break, the rider does not, have to always be observant of distance from others and rewarding.
Reward when it’s his turn to jump and he’s on his own, on top of the reward for performing. Be conscious of this, and he’ll be more conscious of it. He will gradually catch on that he has two opportunities for rewards, not just one.
It may have to start over with each group lesson or schooling day, but over time this does help enormously to have a horse keeping himself quieter. Rider consciousness and consistency is the key.
There are many golden opportunities to help a horse learn quiet separation that riders and handlers routinely ignore, or maybe just not conscious of it. The horse is always aware of separation/togetherness, though, so cultivate that consciousness to match his/hers.
BUT - Being oblivious and not continuously working on separation, during ‘breaks’ or any other time, can undo all the work. Horses have no clue that sometimes rules count and sometimes the rules are on ‘break.’ The horse gets a break - not so much the rider/handler.