When you invite an external user to an IoT Central application, the application doesn't appear on the https://apps.azureiotcentral.com/myapps page until they sign in to the app for the first time.
According to https://learn.microsoft.com/azure/iot-central/core/howto-manage-users-roles:
When you invite a new user, you need to share the application URL with them and ask them to sign in. After the user signs in for the first time, the application appears on the user's My apps page.