What you are looking for are "Difference Mattes."
See this article for the realities of working with them:
http://www.adobepress.com/articles/article.asp?p=1278847&seqNum=2
You can try if you have a clean plate--meaning that you have the locked off shot with nothing in it. But it usually doesn't work out the way you expect--subtle color changes between frames and the moving people probably have similar colors compared to the background so they won't key out perfectly.
Play around with the Difference mode.
Dan Fredley
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