Invitation doesn't work when unregistered users are prevented from viewing all pages.

The basic registration is made for public Wikis. If anyone can register on your wiki, even if everything else is hidden for unregistered, it's pretty much just public. You'll want to complete this default behaviour by using things such as the Invitation Application that's bundled with XWiki Standard.

Check out https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Access%20Rights/#HPublicWikiwithConfirmedRegistration and https://extensions.xwiki.org/xwiki/bin/view/Extension/Invitation%20Application

IMO this would fit better as a forum discussion than a Jira ticket. As far as I see it, it's mostly you needing help to setup your Wiki, and not an issue with the XWiki product. It might be an actual bug, but the description given of your circumstances isn't enough to reproduce the bug or figure out exactly what happens.

I'll close the issue as a `Incomplete` (not enough info), but could probably also close it as a `Invalid` (not about the XWiki tool).