Are you friends with anyone in the group? How did you find it? If you do feel comfortable with at least one person there, perhaps you could ask him or her. If not, how about picking that one person and saying something like, hey next time you're doing ____________ I'd love to be included. I know it can be scary.
Something else I've learned through experience is the more I think something is wrong with me the more I project that. Good luck and don't give up!