Heros - leveling up low level ones ...

When you unlock a hero who is far below the level
of your toon and other heros, are there any ways to
bring that new hero up fast?

add the hero to your group before you get the rewards for any of the quests you do. That way the hero will reccive the xp from the reward.

Use wurms and kill some stuff in the Desolation area. The hero's level doesn't matter much when in a wurm.

If you also have Factions you can level them up quickly doing the fed-ex quests. There's not much fighting, mostly just a lot of running around, so their low level shouldn't matter.

If a hero is below your character's level, that hero will gain more experience from combat. Take them around fighting monsters and they powerlevel very quickly.