Here's how it works:
If you have a high level toon in the group you get diminished xp based on that person's level.

However, this is only for normal mobs.
Elites always give the same xp to your low guys no matter how high your other person is.
This is how instance boosting works.
You level your guys to 8 the normal way doing quests and such.
Then you can start them in instances.

The key to it all is to either group with your high guy and be killing elites in an instance or group without your higher toon cuz it drastically diminishes xp.

Questing rarely NEEDS the 70's help anyway if you're leveling more than 1 person.

Hope this helps