Probably the most important rule of group riding is good communication. Ideally, the leader will explain the ride in advance pointing out any safety considerations such as rough roads, high-traffic areas, easily missed turns, etc.
Speak Up If no one seems in charge, you should speak up and ask, because sometimes there is no formal leader and the assumption is made that everyone knows what's going on simply because they've come out for the ride. This is sure to cause problems. If you don't know, ask, and keep asking until you find someone who can tell you what's going on. There are always a few people who know and are willing to help and it can make a big difference in how much you enjoy the ride and how safe it is.
Pay Attention Once the ride is underway, you can learn a lot about a group by watching the other riders. Try to find and avoid those who wobble and speed up and slow down. These are signs of poor handling skills and possible fatigue that can cause a wreck. Instead, try to ride with the people who hold a steady pace and a straight line because they're less likely to do unpredictable things that can cause mishaps.
Don't Overlap Wheels When cycling in a group, always pay attention to where your front wheel is in relation to the person in front's rear wheel. Keep your front wheel behind his rear wheel, not overlapping it. Why? Because, if he suddenly veers to avoid a hole or rock, his rear wheel will knock your front, which will send you flying.
Use Care When Looking Back Another extremely dangerous maneuver is turning around to look back like you might do if a friend was on the ride and you were trying to figure out where she was. There's only one safe way to look back and it requires a willing helper. To do it, ask the person next to you if you can rest your hand on their shoulder while you look back. They'll say yes and you can then hold on and look back. Having your hand on a shoulder prevents you from swerving as you look back and it keeps your bike going at the same speed as the other bikes preventing slowing that could cause problems. Practice this tactic with a single riding partner before trying it in a group.
Proper Etiquette Finally, there's the challenge of expectorating in the peloton (a bike racing term for what the group is called). Everyone has to do it, so don't hold off. But, do it carefully so you don't soak your ride partners and get banned from the next ride. Always spit with the wind and away from riders. If this means steering to one side of the road first, go for it (but only when you're sure it's safe to move over).