A "texting" app between only users of said app

I wanted to make a texting app, but sending messages directly to other people's phones seems, for one, very complicated. But also, inconvenient. This is not helped by the fact they can't send things back to the app for you.
So.
I want to make an app that prioritizes sending messages between users of it, clean and simple.
...And I don't know how to do that.

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