You will not receive an email or notification stating that your traveler received their app invite.
If the traveler downloaded the app, entered their UserID and Passcode, and accepted push notifications, you will see a mobile phone icon inside the traveler box.
If you click inside the EDIT TRAVELER box, you will see the following under Registered Devices:
An IOS indicates that the traveler is using an iPhone.
An Android indicates that the traveler is using a type of Android phone.
A traveler can enter their app credentials into multiple/different devices. (They could be using an iPhone, iPad, Android, or Tablet.)
If a traveler has not downloaded their app, you will not see a mobile phone icon to the left of their name.
Also, you will not see a "Registered Devices" box listing a phone type.
You can always reach out and check in with your client. Ask them if they had a chance to download the app and gather feedback to see how they're liking it.