Plugin adds a meta box to every post, page and custom post type where users with editor capabilities can promote other users (with author or contributor capabilities, by default) as editors of that post.
Plugin provides the following filters:
custom_editors_enable_roles— Specify an array of user roles which should be allowed to be promoted as editors of specific posts and pages. The default is:array( 'contributor', 'author' ).
A plugin by Metronet.