Problem: You’ve added new members, but they aren’t appearing as options for the “Author” list on the Publish page.

Solution: Go to Admin > View Members and click on the name of the member you want to add to the list.  It will take you to My Account.  Go to the Administrative Options section at the bottom and check the “Include user in PUBLISH page multi-author list?” checkbox.

Alternatively, you can allow all members of a member group to show in the drop-down box.  Simply go to Admin › Members and Groups › Member Groups › Edit Member Group › Members, choose “Member Account Privileges” and choose “Yes” for Include Members in PUBLISH page multi-author list?.