In story settings there should be an option to add co-author/s.
After that every co-author should get confirmation message on their writer portal, to confirm that.
Furthermore I suggest to make it final - only co-author can remove himself from his own writer portal.
If someone is added by mistake, only Episode Support could save this confusion after checking out if that co-author is really added by mistake - so that one author can never remove another.
Co-authors should have access to that story from their own portal.