For all outgoing messages (not for invoices):
Go to Setup (via the 'gear' icon in the main menu)
At company details, look for the field "BCC Email".
Enter the e-mail address on which you want to receive the BCC
For Invoices:
Go to Setup > Billing.
At the top right you can enter a BCC e-mail address.
Enter the e-mail address on which you want to receive the BCC.
It is possible to add multiple email addresses. These must be separated by a comma (with a space after the comma). Please keep the number of email addresses limited to ensure successful delivery of messages.