Before I rode in the street for the first time, I plotted out a path of sides roads and drove it. That made me feel like I had a little more control and knew what was coming. However, riding with others is also a great way to learn. I agree with the thought of, can he take you out for a short ride first before you ride with the group? That may help your nerves. The advice I got of riding the roads more before my first century was some of the best advice. Many of the roads in my century I would have never ventured on my own, but with a group--felt we were more visible and I followed their lead.

Don't go if you're not ready--but you'll have to make the leap of faith eventually (the other benefit is, if this is a normal route they ride, cars will be more aware of the presence of cyclists. This helps also)