In a race, you are often surrounded by lots of wheels ridden by people you do not know or trust. You'll probably never trust many of them, so you have to look out for yourself. The same goes for group rides. If you're feeling a little squirrely, then back off a little. It's better to open a gap or overlap a wheel a little than use the brakes, just know where you're going. Finally, I would take more rides with the people you trust so you can get to the point where you are relaxed, enjoying the scenery, and not paying attention to the wheel in front of you. It might take a while, but once you get there, it will become second nature, and unfamiliar groups will be less anxious. Best of luck.