Rule 9: Yelling
As I said above, yelling is a big no-no. You don't see the pros riding around Europe on their preseason training camps yelling "Carrrrrrrrr...Hole...Gravellllll...Red lightttttt!"
The problem is this: When you are more than two riders behind the person yelling, all you can actually hear is a general "Blurrrrr!" being yelled. So while everyone should be keeping their eyes peeled for general speed changes and obstacles, suddenly the majority of riders are looking around wondering what the obstacle is that has just been yelled out.
No one actually knows if you have just yelled "Hole!" and have not pointed it out, meaning some riders are scanning the ground left, right and center looking for an imaginary hole. Other riders are craning their necks thinking you yelled "Car!", while yet more riders are looking behind them thinking you yelled "George has a flaaat!" Yelling is strictly forbidden!
Rule 10: Slowing and Adjusting Speed
This is probably the biggest crash causer on group rides. For some reason, when someone slows down ahead of them, a lot of riders jump for their brakes and yank the heck out of them, almost skidding and taking everyone down with them.
You should be riding ever-so-slightly to the side of the rider in front of you, so when they slow down you either stop pedaling and start to slightly overlap your front wheel with their rear wheel, or you touch the brakes gradually, once again using the "wheel overlap" as a buffer zone so as not to slow down too suddenly for the riders behind you.
These may seem like a pointless bunch of snotty, European, old-school rules, but they come from very simple principles of general safety for a group ride. So stick to them, and spread the good word to your fellow newcomers to the sport.
For any pro rider worth his salt, these are not even thought of as rules. They are instinctive and are a natural part of riding. This may by why some road riders can come across as rude and arrogant. Ride etiquette is so second nature to them, that in their eyes the only reason anyone would break them would be on purpose.