I haven't read the whole post yet but if it hasn't been said. Most times when I trail ride I'm the leader or second horse on the trail, unless I'm out for fun with people and a leader I trust. From that point I understand how important it is to know ever rider's ability before I start a trail. I always encourage people to tell me if they've got concerns or requests and to let me know what they are comfortable with and capable of doing judgement free so the trail can be planned accordingly. I've had several instances where a rider over estimated their ability and ended up ruining the trail for everyone else because I refuse to leave a rider behind and we cut it short to go back with them, or not telling me something that I should have known that resulted in many people getting injured. I also suggest you know the level of trail you're going on, I have had people complain that I'm moving too slow because I'm being considerate to those who haven't been on the trail before, or want to take in the sights rather then race by everything. I'm more likely to slow a trail down for a single rider than speed up for many.
Telling the trail guide and the people around you of ANY medical conditions is also a must. I'm epileptic so I let riders know that we may have to stop and rest if I feel a fit coming on, some rides involve diabetics, arthritis, epilepsy, or even certain allergies so if everyone is aware then there will always be someone to help if that rider needs it.
The two lead horses are always the 'guards' if we're crossing a main road, dirt road, or water way one horse on either side making a clear safe path to hold off any traffic or to show the safest path through in the case of water. When everyone has crossed they wait on the other side and wait for the guards to take the lead again.
Make sure to have experienced riders throughout the group with walkie talkies or within calling distance so if someone in the back needs to stop I know not to keep going and there is no unneeded yelling in case it spooks a horse.
Let people know about medical conditions
Never be afraid to let the leader know what you can and can't do, they will usually work with you to make sure you enjoy the ride
Be prepared to help someone in crisis, I can't count how many times I've helped a rider or needed help because of my medical condition
Dress accordingly, even if there's a minimal chance of rain it doesn't hurt to bring a rain coat
Most of all just enjoy the ride, if you're high strung and can't enjoy the trail and sights you'll ruin it for everyone.
Show me a horseman who hasn't fallen and I'll show you a man who has never truly ridden.