In the WordPress.org forums, whoaloic asks:

I have a site with multiple authors.
I would like to allow authors who create a post or a page to give rights to other authors.
For now, only administrator and editor can do that.
Is there a solution?

Yep, totally doable. By default, Co-Authors Plus defaults to ‘edit_others_posts’ as the required cap for changing co-authors. With the following code snippet in your theme’s functions.php file, you can make that cap ‘publish_posts’ instead (which authors and above usually have).

/**
 * Filter the Co-Authors Plus current_user_can_set_authors() function
 * so that users with 'publish_posts' can set Co-Authors
 *
 * @author danielbachhuber
 *
 * @see https://github.com/danielbachhuber/Co-Authors-Plus/issues/8
 * @see http://wordpress.org/support/topic/plugin-co-authors-plus-allow-authors-post-to-give-access-to-other-authors
 */
add_filter( 'coauthors_plus_edit_authors', 'db_filter_coauthors_edit_cap' );
function db_filter_coauthors_edit_cap( $cap_result ) {
	global $coauthors_plus;

	$post_type = $coauthors_plus->get_current_post_type();
	if( ! $post_type ) return false;
	
	$post_type_object = get_post_type_object( $post_type );
	return current_user_can( $post_type_object->cap->publish_posts );
}

In preparing this snippet, I also opened a couple of issues in Github:

• Allow authors to set co-authors by default – I’m not sure why we restricted it to editors and admins originally. Also, the existing filter could use cleanup.
• Don’t show search box if user doesn’t have capability to change co-authors – We should have a “locked” UI if the user doesn’t have the appropriate cap