Contact management can be accessed from Customers -> then from the list of customers. Locate your customer and go to the toolbar to the right, for the selected customer click on Other > Contacts.
This option allows invoices, reminders etc to be sent to one or more persons.
From here you can view, edit and create the data of the new contacts of each customer.
Click on New contact
Enter the contacts full name
Contact number
Email address
Select the documents they require
Save