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:
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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