As long as you feel like you could find your way back home if you did get dropped, I'd tell them not to worry about losing you and that you are more comfortable riding in the back of the group. Since it is a pace you feel comfortable with, you can probably keep up and manage not to get left behind.

If they aren't ok with that, or you aren't on the chance you do get left behind, then I'd find a different group or at least wait until I got more comfortable riding, could make signals, and generally feel more secure in my riding skills.