I've been cycling for a number of years, long enough to have encountered a variety of riding situations and to have matured some of my own group riding skills. I know from experience that riding as a newbie in an experienced group can be intimidating. I also know that inexperienced riders can pose a real safety threat within a group. So... I offer these group riding tips that I've picked up along the way, and hope they'll be helpful to others.
No matter how large the group you'll tend to fall into single, double, or perhaps even triple-file lines. It is critically important that you recogize the structure of the group and fall into place accordingly. Cyclists who ride "between the lines" will almost invariably be overlapping their wheels with other riders, and any wheel contact will take both riders down. Maintaining your line becomes especially important as you round corners, as you don't want to cut other riders off in a curve. Perhaps more than anything else, riding safely in a group means riding predictably. If you hear someone behind you yell, "Hold your line!" know that they're doing you a favor, even if it may not sound like one at the time.
To move safely within a group you must be constantly aware of the riders around you - ahead, behind, to your left and your right. When I ride solo I always ride with a mirror - I want to know what's coming up behind me. In a group of more than a few riders, however, I find that the mirror only helps on one side, and what I really need to know is who's immediately around me. In this situation glancing over each shoulder can keep you better informed of who's where.
You also want to pay close attention to the rider in front of you. If he or she is riding irratically then you may want a larger gap between you than if the rider's a long-time riding partner.
When you're in a large pack only the first few riders can see the road. It is more than just a courtesy to point to and call out road hazards such as gravel and potholes. You should also announce when you're overtaking slower riders and if you're braking unexpectedly ("Slowing!"). Good group communication keeps everyone riding safely.
I sometimes call out approaching traffic ("Car back!", or "Car up!") but do so only when it's out of the ordinary, such as a car pulling a wide trailer or a dump truck or the like. Announcing every car causes riders to become desensitized to situations which truly warrant their full attention.
When you're moving in or out of a line, point to where you're going so that riders behind you know what to expect. This is part of maintaining awareness of the riders around you and riding predictably.
When you're drafting you gain about 1% efficiency per mph. You can ride in a group at 22-24mph at about the same efffort it would take to ride at 20 mph solo. To ride efficiently the group needs to ride at a steady pace and avoid unnecessary slowdowns. Two common bottlenecks are:
If you're in the front while descending a hill, you need to remember that if you're coasting then the folks behind you are braking. As you approach the bottom of a hill you should accelerate to maintain your pace as you climb up the other side (otherwise the group will "bunch up" as the faster riders in the back catch up with the slower, climbing riders in front). I'm surprised at how many experienced riders simply don't know how to attack a hill. It's actually easier if you can maintain your momentum by accelerating at the bottom, and it keeps the group from compressing.
It's only natural to slow as you go around a corner, but this can have a cumulative effect with a large group. The trick here is to cut a line through the turn which the group can follow and then accelerate smoothly as you go out the other side.
One other situation to be aware of is when you get out of the saddle for a climb or sprint. It's easy to slow down slightly as you stand on the pedals, and this can be just enough for the rider behind you to collide with your rear wheel. You should practice accelerating slightly as you come out of the saddle to compensate. Since most riders aren't aware of this, don't feel bad about mentioning it when you see another rider do it.
Remember that drafting is much easier than pulling, and it's common for folks to feel the need to demonstrate their fresh legs when it's their turn to pull. Pay attention to the pace of the paceline. If the group is maintaining, say, 22mph on the flats, then that's the pace you should pull when you're in front. If you do choose to push the pace, try and wait until the prior leader (who's falling to the back of the line) is in line and back up to speed.
The whole idea of the paceline is to share the load up front. If you have more than a few riders then you should limit your pull to a half-mile or so. Don't feel bad about taking a shorter pull if you think most of the group is stronger than yourself.
It's generally considered polite to switch off at the top of a hill. This provides for better visibility to ensure you're clear of traffic, and it lets the next leader start out in more favorable conditions.
This one's a bit of a pet peeve of mine, but some riders feel compelled to sprint every hill regardless of the pace of the rest of the group. These same riders will often then catch their breath on the flats, causing the group to ride faster, and then slower than what they'd otherwise choose. I realize that some riders are going to be stronger climbers than others, but if you have extra energy to burn I suggest you take a longer pull at the front. Otherwise, try and maintain your relative position in the paceline.
I ride with aerobars because I like having the additional hand positions. However, using the aerobars limits my bike handling; I'm less stable and I can't brake or shift as quickly. When I'm in a group I only use my aerobars if I'm in front pulling the paceline or if I'm in pursuit mode trying to close a gap. There are some folks that feel you should never use aerobars in a group because of the reducing handling.
This is especially true when you have a small group. You want to make sure that riders who are dropped don't end up lost. You also want to ensure that they're not having mechanical or physical problems.
Occasionally you may notice a situation where the lead rider in a paceline switches off almost immediately after pulling through. That is, almost as soon as the previous leader falls back, the new leader will likewise pull over and start to fall back. As other riders repeat this sequence, you end up with two parallel pacelines, with the outer line going a little more slowly than the inner line (since the riders in the outer line are all falling back). This is called a 'rotating echelon,' and it allows a group to move very quickly, since any one rider is pulling the line for only a short time. This seems to work best in groups of 10-14 riders and will quickly break down if anyone is unfamiliar with the protocol. If you're in a group that starts an echelon, just do what everyone else seems to be doing (like the way most of us learned to dance in junior high) and enjoy the fast, steady pace.