These users will only receive notifications if they have set up their Android device on their Jenkins user configuration page, and have an appropriate app installed. If a "Device token" has not been entered, that user will not receive notifications.
Jenkins will retry once to re-send the notification if the Google Cloud Messaging servers cannot be reached when the notification should be sent. GCM will then store the message and forward it to the appropriate device when it comes online. i.e. The Android devices should receive notifications sent, even if the device happens to be temporarily offline.
See this plugin's wiki page for details on compatible Android apps.