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Create a new Movie Clip symbol. Calling it Scrolling Background or something. Create another symbol for your background characters (for this example, just put all your characters inside this one movie clip) and call it Background Characters. Drop an instance of Background Characters into your master Scrolling Background movie clip then motion tween them across the stage. Then add an instance of Scrolling Background to your stage. That movie clip should repeat over and over, constantly repeating your characters going across the stage. (You'll have to make sure the motion tween of your background characters moves far enough to scroll them completely off the stage and starts far enough over to have them off the stage at the start.)
Hope that gives you a good start.
"Every generation sees the past though the lens of its own time." - Thom Hartmann