From the left-hand navigation menu select the Customer menu item:
This will bring a list of the customers for the business. On the top right corner of the table is the round button to add a new customer. Click on the button to bring up the dialog for adding a new customer.
Enter the name and email of the customer and select if you want to send the invite email to the customer with the login credentials. Click on Save to add the customer.
The example of Welcome e-mail can be found in Quick start guide for your customers.