Push Notification Server

I'm currently learning about push notifications in some Android SDK training. I'm wanting to learn how to integrate a push notification server into an android app. I need help building a push notification server and an Android app compatible with the server. One half of users need to be able to log in as administrators, the other half need to be able to log in as users and subscribe to a group the administrator can send push notifications to. When an administrator sends a push notification to their group, all users subscribed to that group get a push notification on their phone that displays a text message sent from the server. Character limit needs to be around the size of this paragraph. Flow: Create push notification server > admin creates group > subscriber puts in a code to subscribe to a group > group appears in a list for subscribers to toggle notifications / delete > push notifications appear on user device > click on notification to be taken instantly to the text message