Recently external sharing of Power Apps has been enabled.
How to enable external access?
You can simply supply the email address of the external user in the sharing scren of your Power Apps. Then the user will receive an email with a link to the app.
When you add the user you can however not make the external user a Co-Owner. This will make your users only consumers of the application rather than someone who can edit the applications.
You will now need to make sure that as an organisation you check which app have external users accessing your applications. To help you with this as an administator you can use the admin center or you could use PowerApps PowerShell.