mattdu

This plugin uses the custom taxonomy features of WordPress to create “bylines” — essentially, tags representing authors who contributed to a post. The differentiating feature of this plugin is that it should not require any modifications to your theme files (assuming it is already displaying the post author).
This plugin would be useful for publishing situations where the majority of your content is from contributors who you don’t necessarily want to have access to your back-end. It still conveys the appearance that the authors are members of the site (it provides a built-in archive page for each Byline). If your theme displays tag descriptions, you could use the Byline description field to identify guests vs. “staff” writers or provide other author information.
You can see an example of this plugin at work at http://thedailycougar.com

Byline

The Byline plugin is simple and suitable for multi-author WordPress sites.

Best For Multi Author

Solves the co/multi-author problem without modifying the theme. Uses a custom taxonomy, "Byline," that replaces the Display Author.

leticia

This very simple plugin allows multiple authors to be assigned to a Post or Page.
In admin panel, when you add or edit a post or page you can:
<ul>
<li>
specify additional authors, by selecting and adding them in the “Co-authors” panel. (See Screenshots #1)
</li>
<li>
put the shortcode “[sp-authors]” to display the list of co author’s links separated by comma.
</li>
</ul>
When the post/page is displayed to user:
<ul>
<li>you can see the list using the shortcode that can be added in the add post page</li>
</ul>
In any template:
<ul>
<li>
you can user the template-tag “sp_authors_display” to show the list of coauthors “display_name”, separated by comma
</li>
<li>
you can user the template-tag “sp_authors_photos_display” to show the list of coauthors avatars
</li>
</ul>
In search for author functionality:
<ul>
<li>the search take in care all authors, no just the user who write the post/page</li>
</ul>

SP Authors

This very simple plugin allows multiple authors to be assigned to a Post or Page.

Byline vs SP Authors