Push notifications are sent when content is published
If the content is publicit will only notify users that have already interacted with the content. If the content is available to users in groups with content access, it will notify all of those group users.
Users need to enable push notifications for your app on their devices in order to receive notifications.
For more on when notifications are sent for astory or stream go here.
Custom notifications can be sent to all the groups in your account.
Send notifications to your groups
In Groups, select the notifications icon (top right).
Create your notification and select push notification (sent to all users who have enabled push notifications on their devices) or email notification (an email is sent to all active users in a group).
You will see a preview of your notification and a status confirming when it has been sent.
If you have multiple platform apps, you can choose which app your message should be sent to.