have you tried either riding in the very front or very back of the group? my mare used to do this when she was young (2 and 3) but since she has gotten older she will ride where ever.  your horse may just need to be seasoned... lots of miles.  have you tried separating your horse from all others and maybe even leaving it tied at camp?I have a 2-year-old now that I am sure I will be dealing with this with him.  he is very attached to my mare and throws a fit when I leave him at the trailer and ride off from him.  Good luck!

Can you completely separate them at home? Separate to the point where they can't see each other? If so that will be good practice also, and somewhat safe(for you anyway) I have seen them injury themselves before on a fence or barn or anything they might run into.

I also firmly believe in proper round pen work. It builds the horses confidence in you and himself. Lack of confidence is cause for the anxiety.

When I used to do endurance and CTR my new horses would go crazy.  You trailer 6 hours, park in a camping area with 100 other horses, tie to the trailer for the night. Pour the calories on for the long ride ahead. Saturday morning those horse were powder kegs ready to explode. And it wasn't just mine. It amazed me to watch 60-80 horses leave a parking lot at the begining of the days ride. Lots of folks spinning around and usually somebody on the ground.

I agree with the posts above, you need to have control of the horse. Practice controls by your self at home. Get the horse so it will respond 100% of the time to your request. Whether they be sidepass, a head down queue, 360 turn, Laterial or verticle flexes, or what ever you want to work on.  When you get into the group and the horses get excited, start schooling the horse. Ask for the commands that you have taught at home. Keep his mind on you and what you are asking him to do.  Every time you sense he is getting distracted by the group, ask for something.

You may have to work at it for a mile or two, but the horses usually settle down and go to work. It always amazed me to watch the April rides, how excited all the horses were, by June, it was just off to work for the horses, and only those who had just started for the season or who didn't work at home with their horses were still fighting them. The horses seemed to learn they were in for 8 hours of hard riding and learned to save their energy, not waste it prancing around and fighting with their riders.

I had several horses that just could not stand to not be the leader. If they could see a horse butt up head of them, they thought they had to catch and pass it. They would loose their smooth jog or gaits and jigg and bounce all over trying to get going so they could catch the horse ahead. If I let loose of the reins at all they kicked it up and gear and started moving. After a bunch of conditions rides and a few competitions, They would get so legged up and in shape that it was miles before they would calm down.

We knew this horse for two years before we bought him and knew that in every situation he was calm and relaxed with his previous owner.

When we got him home, however, he went totally nuts. He became an unmanageable, anxious horse. We were all floored by this behavior. No one could believe this was the same horse that a few months earlier was packing beginner riders around. He would scream and whirl and pace and just lose his mind when it was time to move horses around the stable - i.e. from the barn to the turnout and vis versa. He would plow over you when being led, he would rear and it was a total turn around to the quiet docile horse we knew.

Long story short, we have found that it simply took several months for this guy to settle and to look at us (his human handlers) as part of his new herd, and to trust us as good decision-makers as far as his survival was concerned. At first, he only saw other horses as safety and protection - not us new humans. When he finally realized that we could be trusted, then we also gained his respect, and he became the quiet, gentle gelding that he had been. But this process took nearly five months. He had been a show horse all his life, and now we upset the apple cart by putting him in a totally new setting (family farm, pleasure riding). This new routine upset him until he got used to it. And as someone mentioned about, could take much longer.

Give him time, give him a routine, and give him a reason to trust that you are a good leader.

I bring this possibility up because a friend of mine has a horse that trailers like a dream - as long as she's by herself. If you put her in a trailer with another horse, she goes ballistic and will do all she can to get out of the trailer to the point of injuring herself.

We found out that she'd been brought to the East Coast in a stock trailer with several other horses and was trampled in the trailer on the way. This experience has traumatized her, of course.

So perhaps there's a traumatic memory in your horse's mind that is triggered by being in a large group? Just a thought.

My horse gets anxious in unfamiliar groups when he's in the middle with horses all around and starting out. Putting him at the back so that he doesn't have to watch his haunches from strange horse attacks works perfect. It only takes him a few minutes to sort things out, but he needs to have an out (either side or front or back) in the beginning.

He can be anywhere in the string after the first few minutes and he's a babysitter horse in that young horses or any horse for that matter can plow into his butt - but he seems to need a few minutes to check things out.

Wet saddle blankets will fix the problem.  Starting with smaller groups of 5 - 7 horses will help.  Camping out in larger groups is also good. 

Tuffy is my 7 year old QH Gelding, and when he was just starting out (coming 3), he had a terrible aversion to mules...he didn't like the smell, sight, or sound of them at all. 

We had an impromptu ride started out with about 8 of us...mostly QH's but one older guy with two mules...one being ridden and the other trainee being ponied.

The only time Tuffy ever offered to try and get out from under me...he crow-hopped and bunched up a bit when I tried to stick him in behind the mules on this trail ride. 

I asked the muleskinner if he'd allow me to desensitize him for future rides...he had his mules stand for a closer inspection, then we rode around them several times, in front and in back of, until Tuffy was "bored" with the whole thing. 

After that, no problems.