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. Your contacts who have WhatsApp accounts will automatically appear in the Favorites screen on your iPhone. 

If you would like to add someone new to your Favorites list on WhatsApp, simply enter their name and number into your iPhone address book.

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:

• + [Country Code] [Full Phone Number].
• Also be sure to omit any leading 0's from the phone number.  
• For a detailed explanation on how to add international contacts, you can read more here.

If you are using iOS 6, and are having trouble with Contact Permissions, please see this FAQ.

Learn how to add a friend on: Android | BlackBerry | Nokia | Windows Phone.

Cheers,
WhatsApp Support Team