How do I expose (crawling) secret articles to web documents?

Google is allowed to register sitemap.

If you enter the Google Search Console and register the site, you can see the following menu. From the Sitemaps menu, you can register a site map in XML format and have the Google search engine read.

I think it will be possible if you register url of the articles created only in sitemap without giving non-members accessible routes.

There may be cases where non-members search through sitemap and look at the posts one by one.