In the sending screen for your invoices and quotes, you will find a Cc field in which you can search and select the contacts to whom you would like to send the email.
You can also directly add a contact from there by clicking on Add contact. Adding a contact here will automatically add the contac at the client level too.
The contacts mentioned in the "To" field will receive the email separately (one separate email for each contact). The contacts listed in the "Cc" field will be Cced to the email you will send to the other contacts mentioned.
At the client level, you can define that a contact should always receive the quotes and invoices you send to the client. Simply tick the corresponding box in the popup to add a contact for the client (under Clients > Contacts). When you send an invoice or a quote to this client, the contact will then already be mentioned in the Cc field (and you can also remove them from this field).
