Office Groups are the center of many team collaboration things. You can add guest to an Office Group and share the OneNote notebook and document library with the guests. Sometimes you want to share 1 folder or 1 document with an external user. Or you want to share a document with an anonymous link.
You need some PowerShell to do this. All Office Groups live in their own site collection so we need to modify the sharing options of the site collection with these statements:
Connect-SPOService -Url https://tenant-admin.sharepoint.com Set-SPOSite -Identity https://url-of-the-office-group -SharingCapability ExternalUserAndGuestSharing
The options for ShareCapability are:
More details can be found in this article.