Used in common speach in California to indicate posession by a group that the person you are communicating with is part of.
see: your guys's / you guys / your guys
Just like "your guys" his is a way to avoid saying "yall, 'yous-guys', 'you people' but mostly "yall". If you're in California and you say yall you'll be looked at like an idiot.
As opposed to "your guys", "yer guys's" is lower speach and is somewhat derisive.
"Those are yer guys's rules, not mine."
"I don't what all yer guys's deal is."