How does opt-in work if you have a free account?

Hello!
I’m helping a client with her first eNewsletter, and trying to figure out how the opt-in works with a free account. I am fairly confident we have to get an opt-in from people to add them to her eNewsletter list. If she has a free account, how does that work? I know we can’t customize the confirmation page or create a workflow to send a thank you for signing up, right? 
Has anyone done this on a free account before? Do you know what the default page looks like? Any pointers or tips?
Thanks!