This strategy has been proven to work to solve this problem.
First thing is that when you are forming your group, make sure you don’t invite party members when you are in overflow. You could be inviting people from different servers.
Before entering the instance make sure you are in the normal zone. DO NOT ENTER while you are in overflow. Once all members aren’t in overflow you may enter the zone. If you are all in front of the portal, make sure you can see everyone’s portrait to ensure everyone is not in overflow.
You may enter the instance while other members wait to hit the “travel” button to leave overflow. DO NOT LEAVE THE INSTANCE, if you leave the instance then you will be put in overflow and will need to wait to hit “travel” then enter the instance.
When doing multiple runs of the same instance, you will need to wait every single time to get out of overflow. If it’s late at night and there isn’t an overflow you may keep running in and out.
IF YOU ARE EVER IN OVERFLOW YOU NEED TO WAIT. Solves this problem every single time.