I've never really used magnitude so I'm not sure how it works with Position.
The magnitude property of a Vector3 value allows you to determine the distance between two points. I'd do this by iterating over transparent objects named "HumanoidRootPart" that are in a range. It's not really efficient, so I encourage you to come up with a more efficient function that gets the job done.
Example of magnitude:
(rootPart1.Position - rootPart2.Position).magnitude
Happy coding!