Here in Los Angeles there are a bunch of groups like that. I'm a member and play bass uke for The CC Strummers out of the Culver City Senior Center, about 90 of us, and from 20 to 60 have performed a variety of gigs. Just yesterday we did the 1st Annual Los Angeles International Ukulele Festival in Torrence, California in the South Bay. If you go to YouTube and search for CC Strummers you'll find a whole bunch of videos.
I was also a member of the advanced instrumental group, the Westside Ukulele Ensemble in West Los Angeles, they also played yesterday. They have about 10-12 members and do a variety of three part instrumentals. They have some videos on YouTube as well. I ended up dropping out, bit off more than I could chew.