Then you need to make a mask in the shape of the person. You can do this by making a copy of the footage of the person and roto the clip to show just the person. Then select the particle system, add an image mask and drag the roto'd person element into the mask.
I had something similar to this that I needed to do for a clip. I wanted to highlight the light parts of an x-ray that was hand held during a shot of a doctor and patient. I knew I needed to make a mask, but I didn't want to have to roto the whole thing, even though it was only a 15 sec clip. So, I made a duplicate of the video, turned it black and white and adjusted the contrast, etc. Then, because it wasn't perfect, I added a soft shape mask to the image so that the layer was just this small area of the x-ray as a mask. Long story short (too, late) I had a moving mask so that I could add a glow effect to one area without adding it to the whole image or roto'ing anything.
"If God could do the tricks we can do, He'd be a happy Man." - Peter O'Toole - "The Stuntman"
Machine Model: Power Mac G5
CPU Type: PowerPC G5 (3.0)
Number Of CPUs: 2
CPU Speed: 2.5 GHz
L2 Cache (per CPU): 512 KB
Memory: 4.5 GB
Final Cut Studio (Not Universal, yet)