How to add rel=author when you have "multiple authors" in one site?
This is simple.


Add a text field to the user profile at /admin/config/people/accounts/fields and call it Google Profile (or similar).
Go to the Metatag configuration page and look up the token for Google Profile.
Add this token to the Author Metatag for the relevent content type(s). The token for my setup is: [node:author:field_google_profile]
Have every author load their Google+ profile to this field in their user profile.


[EDIT] It's worth noting that Google no longer uses rel=author as a ranking factor, so this discussion is now purely academic.

Regards

David