Ah. Parties for me go one of three ways at this point, really. 1. I know the host(ess) and the majority of the people at the party. 2. I host the party. 3. I'm not at the party (as in, not invited/not in a situation like what you describe above). Not quite sure how to bridge that gap.
I met this friend through my sister, actually, and she was my sister's friend first. So I guess I would suggest going another rung further out in your social circle. You know, instead of hanging out with friends, hang out with friends of friends and kind of brachiating from there. It does sound like it's kind of a closed group though which is unfortunate. Can you develop new sets of friends via, for instance, work?
Work is a poor example but I get exactly where you're coming from. Most of co-workers are married/older/generally not people I want to see outside of work. Brachiating is a good visual.Can you develop new sets of friends via, for instance, work?