I have been riding for years, and have been a professional guide for trailrides in some of the roughest and most beautiful country on this continent. I have learned that in the end, both the safety and pleasure of myself and my horse depends on OUR skill, calmness, experience and good humor. Mine and my horse's.
All of the advice given on here is entirely correct, and should be not only be followed, but taught and insisted on. However, people are people, and there are those who will be clueless or careless 'till the day they die. It's just human nature. It is my responsibility, therefore, to not put any horse of mine in a situation where they will likely face something they are clearly not ready for.
Never go on a ride where you will be not have an authoritative say in what is going on around you if you even suspect that your horse will not do well around these ignorant or careless people. And if there are more than 3 riders, the you can bet odds are multiplied exponentially that there will be an "undesirable" in the group. And a trail ride of 300? 100? 25? You and your horse has to have the control, calm and experience to do all of your thinking and that of a dozen other people. Simultaneously. There are some horses I just will not take on some rides, it's not fair to the horse. My horse.
So, my advice on trail etiquette? Don't go on a ride where you or you horse's inexperience could possible add to any already out-of-control ... person. :)