WordPress.org

What is the main difference between Co-Authors and Co-Authors Plus?

The most notable difference is the replacement of the standard WordPress authors drop-downs with search-as-you-type/auto-suggest/whatever-you-call-them input boxes. As a result, major bits of the JavaScript code was changed to be more jQuery-friendly. Eventually, I hope to include the ability to add new Users from within the Edit Post/Page screen and possibly Gravatar support.

What happens to posts and pages when I delete a user assigned to a post or page as a coauthor?

When a user is deleted from WordPress, they will be removed from all posts for which they are co-authors. If you chose to reassign their posts to another user, that user will be set as the coauthor instead.