Add chat function to existing app

I will need someone to add a function to an existing app. The app is very simple where you can register as 1 of two types of users (type a and b), upload a profile and search for other users. What I need you to do is to add chat functionality with some restrictions. Basically type a user can only chat with type b user and not other type a users. And before that, type a user has to send a chat request to type b users. He can chat only if type b user accepts request of type a. The app is connected to database FIREBASE, so the same thing will be used for the chat functionality. The source code for the app will be provided. You need to add in the chat functions there. For the project completion you will need to deliver me both a working APK and the android studio source codes.