Where can I see everyone who signed up for my newsletter?

They go to Marketing → Newsletter Subscribers, which is included with every store. Addresses arrive automatically from a newsletter block on your storefront, and you can type one into Add a subscriber too. If the same person signs up twice, they still appear only once.

The page shows Total, Subscribed and Unsubscribed counts. You can filter by status, search by email, use Copy emails, or Download CSV (both work on the page you are viewing).

To stop emailing someone without losing the record, click Unsubscribe on their row; Remove deletes them from the list. This list is separate from your customer accounts, since a subscriber may never have bought anything. It is the audience your Email Campaigns are sent to.