http://span.state.gov/jul-aug2010/eng/46-49-Flash-Mob.html
This article talks about the start and spread of the trend, but then (thankfully) tells me what I have been looking for--they are protected under the First Amendment, as long as it remains peaceful and presents no danger. I'm going to look up and analyze the First Amendment if I can, and I will have to look up ordinances in the town, as the article mentions that some areas require permits for large gatherings, but at a mall I do not think this is a problem, if I have permission from the mall.