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.
A couple and their pre-schooler were unaware their house was on fire until a passerby warned them at lunchtime today. [Stuff New Zealand]
A video in which protesters picketing a London abortion clinic are challenged by a pregnant passerby has gone viral after being posted on YouTube. [The Guardian]
A driver had to be pulled by passersby from the wreckage of a collision between a car and a minivan on one of the busiest entrances into Manchester city centre. [Mancunian Matters]
Window cleaners dressed as the animals of the Chinese zodiac surprised passersby in Tokyo ahead of New Year festivities. [Washington Post]
A bid to kidnap a nine-year-old girl was foiled when alert passersby on noticing shrieks of the victim’s 11-year-old brother, bashed up the alleged kidnapper before handing him over to the police. [Tribune India]