I'm using WP to build a help/training portal for a commercial software product (Product X). We'd like to restrict access to this new WP site to only authenticated users of Product X as it contains sensitive information. Integrating with Product X's auth API is not a problem (I've done this before). However, all the docs/examples related to customizing WP auth seem focused on the WP admin portal, not the actual WP site. Is such a thing supported? If so, links to examples/docs would be appreciated.