WhatsApp will pull all of the contacts in your phone's address book to the Contacts list. All of your contacts that have WhatsApp will appear in your Favorites list. To add contacts, you must share your iPhone's Contacts with WhatsApp in iPhone Settings > Privacy > Contacts.
To add a contact manually:
Alternatively, if you are contacted by someone in WhatsApp who is not stored in your address book, you will have the option to tap Add to Contacts. If someone sends you a vCard, you can add that contact directly by tapping Add to Contacts on the vCard.
If you are adding an international contact, make sure their phone number is saved in the international format:
Once you have verified the number is in the correct format, try refreshing your Favorites:
This will sort your Favorites alphabetically, as well as restore any deleted Favorites.
WhatsApp Support Team