Just confirming, the author picker in the Shopify admin does current only list the store owner and eligible staff members. As a developer workaround, you can set a custom author name through the Admin GraphQL API, or store the display name in an article metafield and update the theme to use it. Neither is as straightforward as entering a guest author directly in the blog editor though for sure.
This Community is mainly for developer questions and troubleshooting rather than product feature requests, but I’m happy to pass this feedback along on our end. Is your main need simply to enter a guest or external author’s display name without creating a staff account, or would you also want a reusable profile with a bio and image?
Yes, the main reason is to add a guest author to a blog post without creating a staff account. I can find a way as a dev but it doesn’t make sense that this feature is missing from the Shopify blog.