I want the system to automatically email invoices to specific customers through triggers.

I need to email the invoices to 2 people. How do I tag 2 specific contacts for this account so it will email the invoice to both people?

To tag a contact, you'll need to visit their profile page, and use the tags menu on the right sidebar to add a tag. Something like "Invoice Email" would be the tag I use. Then, once you have all the contacts tagged, go back in your trigger and select the Tag for "Invoice Email" as the email recipient.