How distribute custom app (Already published)to another RingCentral customer.

I don't know what you meant by saying "distribute custom app", But if your app is a public app, you should provide a login page so that users under any RingCentral accounts can login and use your app. If you want to promote your app in the RingCentral app gallery, then create an app profile and submit a request to be listed in the app gallery.