You can add customer by following the below given steps:
1)Login to the Admin area in WHMCS.
2)In the navigation menu, roll your mouse over Clients, then click Add New Client..
3)On the Add New Client menu, fill in the contact information for your client.
4)If you want an email notification sent to the customer, click the check box next to Tick this box to send a New Account Information Message .
5)Click the Add Client button to save what you entered. You will then see summary of the profile you created.