Hi Dean_Gross. Today, the Site usage page has a pivot on "Shared externally" to show what’s accessible to authenticated external users outside of your organization to give you confidence that sharing is being done properly. That said, there is a lot of reporting capabilities you can do from the Office 365 admin level wihtin the "Security and Compliance Center", plus use of the new Office 365 Adoption Content Pack, and user managment you can achieve via PowerShell using the "Get-SPOExternalUser" command.
And I'll wager a bet that the above is OK as feedback, and you'd like to see more granularity and visuals locally on the Site uage page. This page has just been established, and as shared during the "SharePoint Virtual Summit" and previous "Future of SharePoint" event, usage and analytics are an area of focus for the team. Glad you like where we've started, with more to be shared re:roadmap during the Ignite 2017 timeframe.
Cheers,
Mark