Just go out and try it. It's really the only option. You may be underselling yourself and may be able to hang with the group just fine. Only one way to find out. I think the reason I've been able to rider longer distances and faster, is by getting dropped. If I just rode with the same group and never tried anything faster and got dropped by people who are much faster than me, I'd never progress. In the first months of riding, I joined a group ride that was 60 miles at 23+ mph. I got dropped after the first 5 miles but hey, I kept training and can finish that ride without a problem. I think getting dropped is actually a good thing, it can be a wake up call for some people.