Question: Companion App for Customers only?

I am building an online directory, as part of this, my listers in the directory need access to an app that provides additional tools and information from the directory.

My question is, what is the best way to distribute an iOS app that has no value to users who are not listing on the directory? I don't wan't random users downloading the app as it's useless for them, I've thought about things like TestFlight but it's not really designed for this, it's for Testing.

A steer would be really helpful!