Good news! Following a recent update to Office 365 (currently being rolled out), external users are now able to (mostly) take advantage of the Search functionality within SharePoint Online. Previously, the search box would appear for external users, but typing in it or hitting ‘enter’ would never yield any results, and browser developer tools indicated unauthorized errors (403) behind the scenes that pointed to SharePoint trying to retrieve information from, and redirect users to, the base domain of the tenant (ie. mytenant.sharepoint.com), which external users don’t have access to.
As of this recent Office 365 update, however, the search bar is now found at the top of all pages (for internal and external users), and after hitting ‘enter’ users are directed to a search results page relative to the current site (i.e. mytenant.sharepoint.com/sites/mysite/_layouts/15/search.aspx) that external users have access to. The only portion of search that it still not functioning correctly, however, is the ‘autocomplete’ functionality for external users, as it only displays “show more results” (Image 1) when typing in the box, instead of showing live results. Looking at the browser developer tools, you can see this is due to a 403 issue where the autocomplete functionality is again referencing a method at the root domain where the external user doesn’t have access (Image 2).
Overall, this is a much-needed update to the SharePoint Online experience, and one that should please external users who previously yearned for the ability to search on sites shared with them.