What you should do is introduce yourself to people at the start, let them know it is your first group ride, and ask them for info about the ride. Where people go hard, where it goes easy, if there are shortcuts or regroups if you get dropped, sprints, etc.

You may want to consider just hanging on the pack and letting them rotate in front of you for a while until you make sure you are comfortable with them, but let them know that's what you are doing, and let the last person coming back know that they should rotate back into the advancing line ahead of you.

Depending on the group, it may be a single paceline or double paceline, or just a loose collection of people riding. Some groups will want short pulls in front and some will want longer pulls in front. Don't accelerate when you get to the front. If you are in front pulling and you can't keep up the pace flick your elbow on the side you want the rider behind to come around you on, and slide over. Slow down a bit but keep pedaling and don't slow down too quickly. For continuiously rotating pacelines, as soon as you clear the wheel of the rider you are passing, slide over in front of them. Hold your speed steady and the next rider should come up and slide in front of you quickly. Just watch them for a while first and copy what they do.