People who claim to be "human magnets" just have some slightly tacky stuff on their skin. That's all.
Have you ever cut a potato with a sharp knife that has a wide blade? It can be quite a challenge to unstick the knife from the potato. Smooth surfaces brought together will stick, especially when they are separated by a thin layer of liquid! Pressing smooth objects to greasy skin creates a suction cup-like effect, especially when the subjects tilt themselves backwards as they tend to do. And some people really do produce especially sticky sweat. There's even a condition known as "acquired cutaneous adhesion syndrome."
Isn't it interesting that the human magnets are always hairless?
And why is it that the magnetic attraction cannot pass through fabric? Authentic magnets don't have that problem. James Randi has debunked human magnets numerous times in lectures and on television simply by asking the claimants to dust themselves with talcum powder. They always fail to per-form.