I assume you are trying to 'highlight' or point out this person. There are many ways.
1: You could use an appropriately shaped O from a text font and enlarge it appropriately and if you have to move it a little use track motion.

2 You could use two tracks. Place same video on both tracks. For the highlighted period of time darken the video on track 1 and then use the cookie cutter to cut out an oval around the person thus allowing the 'normal' lighted individual from track 2 to show through. Again it can be key framed to move with the individual.

3. A modification of 1 above. Use an arrow (look at Wing Dingsfont) and place on track one.

There are others as well.....