The User Related Authors feature enables you to add a Related Authors fieldtype, which allows you to search for members and relate as many of them as you wish to channel entries. Additionally, you can assign a primary author among the authors you assign. The member that is first in the list is considered the primary author.
This feature respects the Include members in Author List ExpressionEngine setting for each member group. To create a Related Authors field, you must select a custom fieldtype of User with the sub field of Related Authors.
Important: This takes a simplistic approach and the related members/authors are stored in a separate database table that interacts with the User:Authors and User:Entries template tags only. It does NOT manipulate the native EE author method and things like access/edit/delete permissions are NOT respected. A single regular author will also be assigned to the entry when publishing an entry.