The range difference within a party to get any useful XP needs to be within 8 or 9 levels.
The mobs you are killing must be pink or lower to the lowbie (Red mobs will give 0XP)
If you are thinking of taking your lowbie into the Altar of Evil - which is the best XP over time, they will need to be a minimum level 76 to get XP from *some* mobs, level 77 to get XP from most mobs, and 78 to get XP from all mobs.