What is it called when a group of people in a public place begin to dance a pre-planned, choreographed routine, and begin with one person or a small group and have others join in?

When a group of people in a public place begin to dance a pre-planned, choreographed routine, and begin with one person or a small group and have others join in, is called a flash mob.

Explanation:

A flash mob is an organized action in which a large group of people suddenly meets in a public place, does something a little unusual, and then quickly disperses.  

One of the peculiarities of these flash mobs is that they do not require the support of the media to communicate, coordinate and act together, since their communication works through virtual social networks. Individuals use communication technologies to spread messages on their social networks from friends and acquaintances, who do the same thing until they build a great communication chain that is capable of mobilizing thousands of people.