Filter for "Sub" - subscriptions

As subscriptors are anonymous for me who has created a group i can not send individual messages to that person.

Now this could be solved by adding a predefined user key.

Eg. I know user "AAA" has joined my group, but i do not know his user key, but i know he is user "AAA".
If i could push a message containing "User=AAA", the app should be able to filter out that and bring upp another notification sound and or coloured text etc.
Other users could choose to mute these messages.