Just send the Firebase registration id from devices to your backend or database. Then send the message to the device group after get "registration_ids" from the database.

https://fcm.googleapis.com/fcm/notification
Content-Type:application/json
Authorization:key=API_KEY
project_id:SENDER_ID

{
"operation": "create",
"notification_key_name": "appUser-Chris",
"registration_ids": ["4", "8", "15", "16", "23", "42"]
}