I agree with the others, but I want to add something. I would say that "working a crowd" means to keep them entertained and/or distracted, usually until some kind of event begins, or someone starts a speech, etc. The entire phrase is an idiom. Don't try to use "work the ..." with other words unless you know what they mean specifically. For example, "work a corner" could refer to a panhandler or a drug dealer or someone busking or a prostitute. It refers to a street corner, and it means that someone has set up at that corner to perform some kind of work. Just another similar example.