First off, you should probably know how to ride a bike in a straight line and have some level of fitness before you think of joining a large group of people on a ride. It'll make things easier on you and the people you ride with.

Show up to the group ride. Know your place and know what skills you need to improve. Ride with the people who are at your level fitness wise and learn the group etiquette. Don't go in with an attitude. You are a guest until people know who you are.

Basically, you need to ease into it. Understand that the club has a social dynamic and you need to earn your respect and friends. Just being on a bike isn't enough. Don't be "that guy" who's off in his own world, making things dangerous for others and being a general nuisance. Some groups are more flexible than others about skill level. In my club, we have a 400 person membership and so there is something for everyone, from the slowest to the fastest. Other clubs are smaller and span only a part of the spectrum and will be harder to join unless you fit the group already. With a 400 person club spanning all levels of skill, we can actually take a newbie and teach him or her how to ride. A club of 30 or 40 people will be less likely to be able to do this. Clubs with smaller memberships are usually more dedicated and will be harder to fit in with unless you already share their dedication.