For all outgoing messages (not for invoices):
Go to Setup (via the 'gear' icon on the top right of your screen)
Scroll down to the next setting:
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