Hello,
I have worked on whatsApp before there are two different way to achieve this paid api and free you can decide this. You can check my portfolio for my previously done project.
Waiting for your replay
Thanks
I am software engineer and mobile application developer. I already have a project like you want but it’s just on iOS platform. If you want to I send you the screenshots and/or video of application. Then if you give me a chance I can develop android and iOS app for you.