PublishPress Authors is the best plugin for adding many authors to one WordPress post. With this plugin, you can create detailed author profiles that linked to WordPress users, or you can add guest authors.
Version 3.9 is now available for both the Free and Pro versions of PublishPress Authors.
The main new feature in these releases is integration with the “Bulk Edit” feature of WordPress. This gives you the ability to quickly add authors to posts.
Using Bulk Edit in PublishPress Authors
On the main “Posts” screen, select all the posts you want to edit. Then click the “Edit” link under “Bulk Actions”.
You will now be able to see the “Post Author” selection box for authors. Choose as many authors as you want. All these authors will be added to all the posts you selected.
Post Counts for Authors
In version 3.9, we've also added a post count for authors. This is an easy way to see how active each author is. You will also see this post count on the “Users” screen.
In upcoming versions, we plan to add separate counters for each post type.