WhatsApp can find which of your friends are using WhatsApp quickly and easily. WhatsApp reads your contacts' phone numbers from your phone's address book and automatically adds them to WhatsApp.
If you would like WhatsApp to recognize new contacts that use WhatsApp, simply enter their name and number into your Windows Phone address book.
When you are adding contacts, make sure that you enter their phone number in the full international format:
If someone who is not saved in your address book sends you an initial message in WhatsApp, you will have the option to save their contact information by tapping add to contacts.
Alternatively, if you are chatting with an unsaved contact and want to save their number, follow these steps: