So who can view a private site?

There are 2 classes of users that have "view permission":

• site members (invite them or allow them to apply for membership)
• "extra access" users you can define in Site Manager » Public or Private

"extra access" users will be able to view all the content from your site but will act as "Wikidot.com users" when it comes to site modifying permissions (SiteManager » Permissions).

In other words — people that are not your site members and you did not give them "extra access" will not be able to browse your site but instead will see the default landing page.