No, you’re on the right track. In order to receive a newsletter from you, the receiver needs to be a member on Ghost. Though, as a member they can do more than just receive newsletters. They can log in on your Ghost site, comment there, and read posts that you potentially haven’t sent as newsletters (but paywalled on your Ghost site).
On the Ghost(Pro) pricing site you can use the slider on top to set the amount of members you have. This will then show you the available pricing options.