Teamwork - Give a helping hand, watch for problems (loose straps, loose equipment), and help each other to be safe and enjoy the ride.
Be Predictable - Group riding requires even more attention to predictability than riding alone. Other riders expect you to ride straight, at a constant speed, unless you indicate differently.
Communicate - Use hand and verbal signals to communicate with members of the group and with other traffic.
Hand Signals - Hand signals for turning and stopping are as follows: Left are straight out to signal a left turn. Left arm out and down with you palm to the rear to signal slowing or stopping. And, for a right turn, put your right arm straight out (in areas where this is legal) or put your left arm out and bent up.
Verbal Warnings - Along with hand signals, verbally warn cyclists behind you of your changes in direction or speed. The lead rider should call out "left turn," "right turn," "slowing," stopping," etc. Announce a turn well in advance of the intersection, so that members of the group have time to position themselves properly.
Announce Hazards - When riding in a tight group, most of the cyclists do not have a good view of the road surface ahead, so it is important to announce holes, gravel, grates, and other hazards. Indicate road hazards by pointing down to the left or right, and by shouting "hole," "bump," etc., where required for safety. Everyone in a group should be made aware of hazards. However, not everyone needs to announce them.
Change Positions Correctly - Generally, slow traffic stays right, so you should try to pass others on their left. Say "on your left" to warn the cyclist ahead that you are passing. If you need to pass someone on the right, say "on your right" clearly since this is an unusual maneuver.
Watch For Traffic Coming From The Rear - Even when you are occupying the proper lane position, it often helps to know when a car is coming. Since those in front cannot see traffic approaching from the rear, it is the responsibility of the riders in back to inform the others by saying "car back." Around curves, on narrow roads, or when riding double, it is also helpful to warn of traffic approaching from the front with "car up."
Watch Out At Intersections - When approaching intersections requiring vehicles to yield or stop the lead rider will say "slowing" or "stopping" to alert those behind to the change in speed. Each cyclist is responsible for verifying that the way is clear before enter the intersection.
Leave A Gap for Cars - When riding up hills or on narrow roads where you are impeding faster traffic, leave a gap for cars between every three or four bicycles. This way motorists can take advantage of shorter passing intervals and eventually move piecemeal around the entire group.
Move Off the Road When You Stop - Whether you are stopping because of mechanical problems or to regroup with you companions, move well off the road so you don't interfere with traffic. It is usually best for the lead rider to pull forward in the stopping area and for other riders to pull in behind the rider in front of them. When you start up again, each cyclist should look for, and yield to, traffic.
Ride One Or Two Across - Ride single file or double file as appropriate to the roadway and traffic conditions and where allowed by law. Even where riding double is legal, courtesy dictates that you single up when cars are trying to pass you if the lane is wide enough for them to safely do so.
Wait At Turns - If the group becomes at all separated, even by a few dozen meters, someone should wait at the turn until the next rider arrives at the intersection, and so on until all riders have made the turn.
Two At The End - For safety and as a courtesy, if the group spreads out, the last two people should adjust their speed to ride as a pair. If either should need assistance they will have a helping hand.
Source: International Bicycle Fund