Can I sign up with an international number?

It is not a problem if you have an international number.
Instead of texting your Join Code to 735-343 you will text it to this number: (512) 308-3535.
Once you have enrolled and downloaded the Waldo Photos app, you will be asked if you would like to turn on Push Notifications.  You can say "Yes" or turn them on in your phone's settings.