You don’t have to add contacts in Whatsapp itself. Whatsapp checks your phone’s adressbook and adds the Whatsapp users based on their phone numbers in your Whatsapp contact list.
If it happens that you know for sure your contact has Whatsapp, but the name does not show in the contact list, make sure you have filled in the international country phone number with the + sign.
If you contact changes his or her phone number you have to update it manually in your phone’s adressbook. When this is done Whatsapp will automatically check the new phone number and add it to contacts.