Hi, I'd also like to say that being able to share an app with any external user is something we'd like. In the same way that a Form can be shared, it would be very useful to be able to create a Power App that does that. It is not always feasible that a user must be added to your tenancy as a guest.
The ability to share canvas apps with external users is now availabile in public preview! The documentation can be found here: https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/share-app#share-with-guests
The blog announcement is here: https://powerapps.microsoft.com/en-us/blog/share-canvas-apps-with-guests-in-your-organization/