The idea behind this is for select instances you can rezone in at a later time to complete the instance without having to start over.
When your instance becomes persistent you get tagged with a unique ID representing the instance. You cannot reenter your instance while grouped with any character that has a different ID for that instance. You can bring people who have no ID for that instance in but they will get tagged with the ID as soon as they zone in. [Post from Silverfrost on EQ2 Offical Forums]