If you signed up for the notification, you will get an email when guard is available to your account.
Note this from the FAQ. It says the same thing I said above, but with a little more detail. Note that they are now specifically saying “ADT pulse“ not just “ADT.”
7. Can I connect Alexa Guard to a professionally monitored security service?
Yes. You can connect a supported security system, such as Ring Alarm or ADT Pulse, to Alexa Guard to arm and disarm your security system by voice. If your connected security system includes professional monitoring services, you can also choose to forward Smart Alerts, including audio recordings of selected sounds, to your security service provider. Your security service provider can determine whether to take-action on your behalf in response to receiving a forwarded Smart Alert. How your security service provider responds to receiving a Smart Alert may vary depending on your provider. Alexa Guard does not offer professionally monitored security services.