How do I register my iPhone to receive email reminders_
Your phone does not need to be registered to receive email reminders, as email is done through your email client. If you are referring to PUSH notifications, then all that needs to be done is to install our iOS app and log into your account.
How about for Android?
It's the same thing for Android. You just need to install the app and log-in and your device will be detected in our server. You can double check it by going to the web app and then clicking to Settings -> Reminders. You should find your device there.