WIthin the admin menu, under the "add user" tab, there is a checkbox to send an email to the newly created account you made. I tried customizing using the "welcome campaign" with the settings set to send the welcome email "When a new WordPress user is added to your site..." with role of "Customer". Currently, if you attempt to set it up this way, it asks the user if they want to subscribe, rather than sending the email I set up, which makes sense given how Mail Poet is intended to work.
For websites that don't allow people to create their own accounts, being able to customize this is essential. I presume the coder would need to hook into the wp_insert_user() function, or something similar.
Cheers!