Push notifications are provided for both iOS and Android using Google Firebase. It's called Cloud Messaging. https://console.firebase.google.com/project/access-my-community/overview (Login: firstname.lastname@example.org)
You can send push notifications from the MCD website. Go to Members Centre, select Tools, Push Notifications.
Select Send a New Push Notification and the following screen will appear.
You can selected the audience who will receive the notification. This can be everyone who has installed tiles from an organisation. A specific tile group, or a single user. Enter a title and the body text for your notification. You can also add advanced options. These are to display a popup message in the app, and also do a Directory or Diary search within the app.
Tapping Send will push out the notification to the recipients.