If you’re Tweening the Part then are you trying to CFrame it?
When using the other types of BodyMovers, like BodyPosition to keep it at a certain height, or BodyForce, to provide a force straight up, you don’t need to worry about keeping the player on it because friction will take care of it.
Use BodyVelocity to move the Part, but the others BodyMovers can keep it at a certain height no matter if a player (or players) is on it.