Is there a way you can make players or entities glow in the dark?
Well, its not really glowing in the dark, but maybe you could use the
Glowing effect (id = 24) http://minecraft.gamepedia.com/Potion_effects
Note: This will only work in the latest snapshots and above.
This will draw an outline around the mob/player, which you can see through a wall(maybe not what you are looking for).