A motley crew is a collection of people you wouldn't expect to see together. If a construction worker, a clown, and a rock star walk into a room — that's a motley crew!
Motley is a word meaning "mixed" or "miscellaneous." It originally meant "of several colors at once" and was applied to speckled or multicolored fabric and clothing. Have you ever seen jester outfits at a Renaissance fair — the ones with the bell hats and the trousers that are half one color and half another? That could be described as being motley. A motley crew is the human equivalent: a mixed bag of people, often working together for a shared purpose.