24reset4_06be2834441This works every time we tried it.
We have figured it out to make it work 100% of the time.
Here is an explanation in 7 steps!

1) Enter the instance alone
2) Invite an friend or just somebody and make sure they stay in your group.
3) If your work is done in the instance, leave it.
4) Once your outside of the instance leave group (right click your name and click leave group).
5) Then invite the same person you have invited before.
6) Enter the instance again and it should reset!
7) Done with the instance run?  repeat step 3 and 6 and enjoy!