Navigate to the side navigation bar and choose "Customers" from customers.

Select the particular customer you wish to add recipient for sending the invoice.

Scroll down the right-hand window to find the "Billing & Contract" tab and click on it.

Choose the "Billing & Invoice" tab from the provided tabs.

In the invoice contract section, you'll find a 'To' and 'Cc' section where you can choose from options such as 'primary,' 'secondary,' and 'other,' which are pre-set defaults.

If neither a primary nor a secondary user is available, you can select users from the dropdown menu that appears when clicking on the checkbox labeled 'others'.