How to query a list of registered user accounts on a website in Java?

I'm writing an app for android and ios mobile devices. The app needs to talk to a website that another developer is working on: My app will have a username/password field that needs to be filled in before proceeding.

I have the contact details of the web developer but I am kinda new to this (getting deeper data from a website, more than just html-scraping) So how do I make a request to a website and get -say- a list of registered user accounts on a website? What kind of info do I need to ask the guy making the website?