As always, I enjoy your posts Bruno!
Just one thing, you are right about informing users about their chances to become customers or not due to their location, but locking them completely out of the site would be a wrong step to do IMHO (not sure if you actually meant that either). There are two reasons: For a travelling customer, who might be outside the U.S., the ability to continue to use the service should be possible. You don’t want to lock your customers out of your service, just because they are travelling. 
The second reason is just for the information. Even though you might be outside of the U.S., it doesn’t mean you shouldn’t be able to inform yourself about the service. For instance, this page should still be available to read for anyone outside the U.S. https://venmo.com/about/product/
So yes, do warn a guest about their inability to join, because the service is only available in the U.S. only and base that decision on GEOIP or any other methods, but I say don’t block the access completely. 
Scott