I'd personally throw the 67 Druid into the mix.
It's the same level as the others, so doesn't need to play catch up.
Also, a Druid gets you a toon who can Tank or Heal down the road, should you want either path.

Then play the five-some as a normal group.
You can PvP, which if the queues aren't too long, I'd highly recommend in x6 or x7 through x9 levels.
You can instance grind, via dungeon finder.
Questing will be decent experience too, especially if you stack quests.