How do I add a contact to the WhatsApp Messenger?
From the Chats pane, tap the Chat icon at the bottom right of the screen.
Select New Contact from the next screen.
Enter the contact information of the person you want to add.
Tap Save at the upper right of the window. The next screen will display the entire contact.
Your new contact will also appear in your address book.
In the Chat pane, tap the chat icon at the upper right of the window.
In the next screen, tap New Contact.
Type in the name and telephone number of the person you want to add — name and telephone number and email are mandatory.
The contact information will then appear in your Contacts.
Read more about it here