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 your WhatsApp Contacts list.
If you would like to add a new contact to your Contacts list on WhatsApp, simply enter their name and number into your phone's address book, then refresh your Contacts list:
Alternatively, 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.
When you are adding contacts who live in other countries, make sure that you enter their phone number in the full international format. The format is:
WhatsApp Support Team