A tool to use WhatsApp for mass communication

Hello, I’m looking for an app/toolkit which can support using WhatsApp broadcast for mass communication.

What I want is to be able to offer subscribers a way of automatically adding themselves to a broadcast list by simply texting a keyword. As well as auto joining, I’d like to offer a second keyword which allows subscribers the ability to unsubscribe. Does anybody know if this exists?

I see a handle full of businesses, newspapers and groups appear to have this ability.