I'll second Kristen's opinion of NOMOTC, with some extras. I, too, noticed that it wasn't about the kids, it was about the moms, and I did not feel welcome there with my little ones. (This was in Austin, TX, 10 years ago.) At the time, most of the mothers of HOM (higher-order multiples) in the area felt the same, so we had our own separate group for triplets and up. We had a yahoo group, and we went out to a moms-only dinner once a month. Sometimes moms got together for playdates, but we didn't do any big activities as a group with our kids. It filled a need to be with other moms "like me" in having triplets or more.
I never did find a playgroup that worked, though. Never found a multiples-only playgroup, and never found one friendly to multiples. We're a walking freak show, not somebody you make friends with, apparently. One lady ran into me with my triplets at a coffee shop once, and invited us to join her playgroup. We went three times, and then the group was scheduled to meet at my house...whereupon I was called the day before the meeting and told I was no longer welcome. I tried a local babysitting co-op, too (don't get me started on finding sitters for triplets!). They kicked me out.
Multiples intimidate other families, I think. Singleton babies don't know what to make of it when multiples come to play. The multiples are already accustomed to crawling all over each other and playing together, and the singletons flip out. Moms of singletons don't know what to make of us, either. Some of them put us up on a pedestal as being goddesses of child-rearing just for staying sane (or some semblance of sanity). Some of them don't understand the chaos that comes with bringing more than one baby anywhere, the 20-minute diaper change routine, the 1-hour feeding routine, the fact that MOMs can't just sit still and watch one child, we have to constantly bounce to keep one child after another out of trouble. Based on my experiences in Austin, it seems like HOMs get the same reaction from twins.
It's hard not to take it personally. But keep trying. Eventually either you'll find the group that fits you, or you'll make your own, or you'll just settle in with some friends who are welcoming without any structure at all. Any way, while you're looking, at least our kids have built-in playgroups!