Typically, your light painting source is very low power so that it builds up over the course of the exposure without over exposing. Doing long exposure with people is hard. The subject would be required to stand extremely still. Any movement will cause the subject to be blurry as they will literally be appearing in multiple places within the image. If the subject moves too fast, they will completely disappear from the image. We use this technique frequently in music videos. I've also experimented with using flash strobes to expose the subject in the split second even though the shutter remains open. At that point, your subject can move out of frame and still be exposed from the flash.
