Hey there,
I remember when I first started my group riding that I was very nervous about riding with them but the group was really accepting and very encouraging. Just let them know that you're new and that this is your first ride. Also, bring a map with you just in case you get seperated for some reason. I ride with a group that has beginner group rides, where someone with experience leads a group of beginners. Maybe your group ride will have something like that as well. Good luck and remember, always have fun!