Kylie Jenner's Model Friends Playing Hottest Grab Ass Ever

More insanely hot action from Kylie Jenner's birthday fiesta -- this time, her smokin' hot model friends are grabbing handfuls of each other's asses ... and catching it all on camera.

Hailey Baldwin, Bella Hadid and the rest of the crew threw on their skimpiest bikinis and headed out on a yacht in Turks and Caicos ... taking turns slapping each other's butts and taking pics.