Hello, I'm reposting the original question and a very helpful response here, as I don't have enough Karma points yet to comment on the post. What Paresh has suggested does work for pages - but NOT if the menu item links to /forum, or /slides, or /blog (I guess these aren't seen as pages?). I tried creating a new page (which gives me the setting Paresh mentioned), but as soon as I change the URL to /forum I get the same problem again.

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Hello, is there a setting that enables me to restrict who can see our website/blog/forum? I'd like to limit it to only pre-authorised users - not just limiting the ability to post, but the the ability to see anything at all. 

The answer I received: if you are using Odoo 14, the Website module allows you to control this via a setting in the Page Properties. To enable this, log in as a user with Website Editor privileges, go to one of the pages on the website, open Pages > Page Properties from the menu on the top left, go to the Publish tab and select Signed In in the Visibility field.