Hi @ITGeek,
I am afraid there is no trigger In Flow could be fired when a user shares SP site to other users who do not belong to your organization.
If you don't want your members share the site with other users, you can turn off the external sharing, the SharePoint external sharing setting on this page is the same as the one in the Microsoft 365 admin center, under Settings > Services & add-ins > Sites, for more information about it, you could refer to this doc:
https://docs.microsoft.com/en-us/sharepoint/turn-external-sharing-on-or-off
Best Regards,
Community Support Team _ Lin Tu
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.