The following tips apply equally to both Android and iPhone. The only difference, of course, is that the iOS interface is a slightly different design. But it’s all still in the same place.
If this is a new contact, make sure their phone number is first added to your Google Contacts app (or your iOS Contacts app), along with the proper international and local dialing code. If you don’t know the dialing code for a particular country, you can find it here. Now restart your WhatsApp app and begin a new conversation in Chats. This will bring up all your contacts who have registered their phone number to a WhatsApp account.