"Passerby" can be spelled alternatively as "passer-by," so, conceivably, you would want to say "passers'-by," though you may just want to find another noun to convey the same meaning without the confusion.
The possessive form of the singular noun passerby is passerby's.example: That's a passerby's dog barking.
The plural form of "passerby" is "passersby."
The possessive form for the noun passerby is passerby's.
The plural form for the compound noun passer-by is passers-by.
passersby
passers-by
Singular possessive: secretary's Plural: secretaries Plural possessive: secretaries'
The plural is relatives. The plural possessive is relatives'.
The plural is lads. The plural possessive is lads'.
The plural form is echoes. The plural possessive is echoes'.
The plural form is suffixes. The plural possessive is suffixes'.
The plural form is Mexicans. The plural possessive is Mexicans'.