What skills are required depends a lot on what type of group ride it is. For example, if it's a friendly breakfast ride, you won't need the survival tactics required to sit in on a grueling racers' training ride.
If you've only ridden alone, it's best to learn group riding in a small group first. The principles are similar regardless of the group size. But, riding with a couple of friends lets you practice basic skills without the confusion that can occur in a cycling mob.
Perhaps the most important technique is being able to ride in a straight line. If you're wobbly, riders will be afraid to ride next to or behind you because it looks to them like you're not in complete control of your bike. Usually, people wobble because they're too tense and feel nervous. If you can relax, especially in your shoulders and upper body, the bike will go straight on its own. Another important skill is maintaining a safe position. Try to keep a safe distance behind the guy in front and never allow your front wheel to overlap his rear wheel because, if he has to swerve to avoid a hole or stick, he'll hit your front wheel knocking you off your bike.
Communication is vital on group rides. Everyone is responsible for keeping each other informed about threats to the group. The front rider alerts followers by voice and by pointing if he spots any obstacle that could cause a problem such as a pedestrian in the road or a pot hole or sewer grate. After he points and/or shouts out the hazard, the next person in line repeats it and so on. This ensures that the people behind, who might have missed the front guy's warning because they're back aways, get the message. Meanwhile, the people in back let riders ahead know when cars are about to pass.
There are accepted signals to use: The cyclists' cry for a car passing, is, "Car, back!" This tells other riders to tighten up and move right to let the vehicle by. Cyclists will also shout, "Car, up!" when a car is pulling out from a curb or slowing or turning in front. Most important is pointing out and shouting out dangerous things you see such as "Dog!" or "rock!" because, if one rider goes down, there's a strong chance many others will, too.
One of the best ways to learn the dynamics of a group ride is to join one. But, let the ride leaders know that you're new and learning. They'll explain the route and any hazards they know about. And, they'll explain how their ride operates. You should then sit at the back of the group for a while and observe what goes on. You can learn this way and get a handle on the etiquette of riding with this group.
It's great fun riding in a group and all it takes is a little practice. Have fun. We can suggest some local rides you'll like.