Don't feel obligated to do something that makes you uncomfortable to stay with the group. It is sometimes the case that a "herd mentality" governs the group, and they may ride in a way that makes you feel unsafe - e.g. blowing stop signs/traffic lights, filtering to the front of a stopped line of traffic, blocking traffic, yelling at other road users, etc. Hopefully, this being a beginners ride, this won't happen. And, honestly, with smaller groups of more experienced riders this type of behavior is pretty rare. But I've seen it happen with larger groups (>12-15 or so) of mid-level riders. Larger groups (> 25-ish) can get pretty crazy (but, again, probably not on a beginners ride). If this type of thing happens, I abandon the ride and do my own ride. Since it's a beginners ride, if you do decide to abandon, you should let the ride leader know, of course.
Otherwise, just don't be a d!ck. Ride smoothly and predictably - no sudden stops, swerves or accelerations, signal your intentions. If you are not comfortable riding close to others while reaching for a bottle or adjusting equipment or something, then drop to the back of the group before doing so. Don't b!tch and moan - about the speed of the ride, the route, the way the leader organizes the ride, etc. You chose to ride with the group, if you don't like it then just don't go back. Do not go off on your own without telling someone, preferably the group leader. Above all, don't crash and take others down with you. You have to extend your awareness and perceptions to cover those nearest to you and the group as a whole, as well as yourself. Pay attention, your riding can now affect the safety and enjoyment of others.