There is a company called The Inflatable Crowd Company that was created for Seabiscuit back in 2002. Ever since then, companies have been using these, but you'd never know it. Each torso is dressed and wears a wig. In the movies, they look just like real people.
You can learn more about this at Inflatable Crowd. I put pictures of these plastic crowds below from the movies American Gangster, Cinderella Man, We Are Marshall, The Changeling and Glory Road.