The cast of "The Wizard of Oz" is an old standby a group Halloween costume. You only need four people, and you can even include the dog if you like.
The most difficult costume for "The Wizard of Oz" look is likely to be the Tin Man. We recommend using cheap silver or metallic spray paint on some old clothes and an oil funnel. Attach a string to both sides of the funnel to make it into a hat and then use makeup to color your face silver.
After you all get done getting dressed, head off to follow the yellow brick road.