Communication Android app development

I want to have a server hosting information & Development estimation for about 500,000 users (3 Lakh). They will be divided into groups of 100 max. Therefore we will have 5000 groups. Each group will have one admin. The users and admin will use mobile phones. The admin will send some information to user in the app, and user will either accept or reject. The user also can make requests to admin. High level requirement is as follows User Authorization and Authentication using Email ID. Send User to User Push Notification of each event. User Module: – Android Mobile App Respond to request by Admin. Send request for Information to Admin Manage user Information. Admin Module: – Android Mobile App Send Information to Group Users Track Status respond to request Mange Group Information. Web Admin Module: Manage Users Manage group Track Status While quoting for the project please mention following details in your message Backend technology & Database you propose Required Server configuration to manage 5,00,000 users without any problem Also mention your assumptions and limitations clearly