Putting English waltz aside, Chinese people in general have a tradition of public gatherings. It doesn’t matter in which city you are (big and modern like Shanghai or small like Suichang) they all have something in common. In each and every one of them you can see people dancing (or practicing taichi) in the evening on streets, in parks, at squares and any other places as long as there is enough space. Chinese people do not shy from public. Anything here has a public aspect to some point (even visiting a doctor – however ridiculous that might sound).
As for the dancers anyone can join them any time. There’s always someone who’s leading the dance. You can just imitate what he/she’s doing. If you’re ever going to visit China try this at least once. It’s a great fun (and gives you wonderful opportunity to interact with people).