I am assuming you are riding a 16mph average as stated in your post. And you said that you had more in you. Group rides have a tendancy of rolling faster than thier category. In our club a B ride is considered the following:
B Rides = TOURING. AMS between 13.1 and 15.0 mph; cruising speeds may reach 18 mph. Uphill pace slows down quite a bit. Rest stops every 10 to 15 miles.
However since this is your first ride with this group it might be wise to go with the C+ group and get to know people and the way they do things.
Take your time and enjoy the group ride.