A passerby is literally someone who passes by something. Usually it is referenced to people who are walking outside of a store as they go on to somewhere else. This term does not require the use of a hyphen.
The correct plural is passersby.
Passerbys is an incorrect formation. However, it occurs once out of every 25 times the correct form is used. This popularity has increased dramatically over the last decade. This suggests that people are loosening their idea of a correct form of the word. We would not be surprised to see this as an approved alternative in the coming years.
As of now it is not approved and passersby should be used for formal writing.