Most groups have a designated person responsible for the trip leading. They decide when and where to paddle, and are responsible for setting the pace or level of the trip, beginner, intermediate etc. Be sure to only go on trips that you are able to handle. It is unfair to others to show up for an advanced level trip when you are still a beginner. Unless of course it is a rescue training session and they need a fresh victim :)
Group trips are usually made up of paddlers who have a broad range of experience levels so they are a good place to learn new skills. Most paddlers are very willing to share information and advice. Especially when it comes to showing off their latest fancy roll!!
Your local paddling shop should have information about the clubs in your area. Most will allow visitors but usually the annual fee to join a club is small compared with what you will get out of it.
Go on a club paddle to find people you relate well with, its a great place to find a network of friends you can paddle with on your own!