The list of "rules" already posted is what I have always understood. Some other things to add to that list:
- when stopped to get a drink (stream, etc.) or stopped because someone dismounts... wait until everyone is ready/mounted before moving off.
- when crossing an obstacle like a stream, wait until everyone crosses before speeding up.
Usually, the "rule" are for groups of strangers. When riding with a regular group, the "rules" can be anything that is agreed upon. I used to ride with someone and either one of us would randomly lope off is a different direction whenever we felt like it (part of the reason it started was to make sure our horses' never became buddy sour). In other words, the "rules" can be whatever the group agrees to.
Another hard and fast rule of riding (and life) is that you can't please all the people all the time. When you ride in a larger group, there will always be someone that is unhappy with something. Too fast, too slow, whatever.
A corollary to that rule is that the amount of complaining is usually inversely proportional to either the amount of training the horse has, and/or the ability of the rider to control that horse.
Best thing is to identity and avoid those people. :)