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Plugins: Co-authors Plus
What it is: “Assign multiple bylines to posts, pages, and custom post types via a search-as-you-type input box. Co-authored posts appear on a co-author’s archive page and in their feed. Co-authors may edit the posts they are associated with, and co-authors who are contributors may only edit posts if they have not been published (as is core behavior).
Add writers as bylines without creating WordPress user accounts. Simply create a guest author profile for the writer and assign the byline as you normally would.
On the frontend, use the Co-Authors Plus template tags to list co-authors anywhere you’d normally list the author.”
More info: https://wordpress.org/plugins/co-authors-plus/