Figure out which person is causing your issue. Have one person leave and the rest of the group try to zone in. If they can zone in, that person is the issue. Once you have identified that, have someone not bugged create an alt. Have that alt invite that person that is bugged. It should ask the alt if they want to take over an instance in progress. Click yes. Drop party. Delete alt. Profit.
Tried this several times, it doesn't work or only seldom works for some people. There is an issue with the system that needs fixing, period.