I’m not sure but as far as I can remember (or at least about 2 years ago) I found a function to activate the subscribers without their needing to opt-in (even though a warning from Aweber would pop up). I lost about 70% of my subscribers in my very small list back then when I changed, but I’ve been able to grow it ten-fold since. And I used opt-in, because I didn’t want anyone to complain to me later.
Email marketing is such a critical component of business for most companies (with the rare exception) because you able to communicate with your audience in the place they spend a good chunk of time (their inboxes). You also have the added benefit of building an asset (an email list) that no external platform (I’m talking to you Facebook) can mess with.
Before you can send AWesome emails with AWeber, you'll need someone to send it to. The easiest way to begin building your list is to import contacts that have opted-in to your email list, or gave you permission to send them emails. These could be subscribers you've emailed from another provider, or a list of contacts that you've built through your business.