In what city?
What kind of dancing do they do? Street, ballet, what?
If you use 快闪 in Chinese and search it in Youku. You may see tons of video there.
In my school in Guangzhou, students, especially who is from Dancing Association, did it several times a year. But sadly I have never knew what kind of dancing they did.
中文说快闪族（kuài shǎn zú),在北京上海吧。