So....have a mage in your party and a lock....

1. Finish dungeon
2. Have your mage portal/hearth to a city.
3. Log out on your other 4 toons
4. Mage resets the dungeon
5. Log back into your other 4 toons
6. They should be back at the start of the instance
7. Summon the mage back (might have to step outside instance to summon)