If you were to log them out, once they reach... cannot enter more instances... that would save time.
This is a warcraft bug, most likely; when five toons do instances together, never alone... and 1-2 cannot enter, yet the rest can... ?

If you had Warlocks (or the old, "Have Group, Will Travel"), that too would save time.

I'm personally leveling teams to 80th.
In as little real time as is possible for me to level them.
I don't want to be 10th level for an extra hour.
But it is real time I care about; logging a toon for an hour, because of the instance bug or leaving them sitting at the zone line and trying to enter every 5 minutes, either way it is the same amount of real time.