As Oakleaf has suggested, each group is different so you might want to show up to your first ride a little early, find the ride leader (assuming there is one), introduce yourself and ask if she/he or anyone else would be willing to mentor you during the ride. Otherwise, ask if they can tell you what to expect.

Every group has a different personality. I love our local club, but I've heard complaints from others that it can be intimidating to new riders (or riders new to the club). Sometimes, we're so engaged in what we're doing--preparing our bikes, saying hello to old friends, talking about the route--that we may unintentionally ignore someone new. So, I think it pays to be outgoing; don't wait for people to welcome you. Introduce yourself instead. You'll make friends in no time.

Whether riding in a loose group or a paceline, you should strive to ride predictably, keeping a safe distance from other riders and maintaining a steady line. Use voice and hand signals to indicate any road hazards, to indicate that you or the group is slowing, coming to a stop, turning right or left, etc. Announce "car up," "car back" and "car around" as necessary. Be mindful of where riders are around you and announce your presence if you're coming up behind someone or overtaking them. Obviously, you should follow the rules of the road. If someone is making you uncomfortable because of their own erratic riding, keep your distance.

Every group is different as to whether there's a no drop policy. If there isn't one, make sure you have a route map or know what the route markings look like. As always, be prepared, even in a group. Have the gear you need to change a flat, adequate food and water, take a pre-ride potty break, etc.

Have fun. Group riding, while not for everyone, is a blast and a good way to make friends.