You can add new contacts to WhatsApp manually from the Chats tab or you can scan someone's QR code within the app.

Are your WhatsApp contacts suddenly not showing up on your Android phone? Don’t worry — this is a common issue, and it can usually be resolved in just a few steps. Whether it’s due to permissions, ...

If you don't want certain numbers or contacts messaging or calling you on WhatsApp, you can block them by adding them to your block list.