Ghost sends newsletters natively: every post can be emailed to your members, free or paid. Three things to set up first.
Turn on members
In Settings → Membership, choose who can sign up. Every theme in the collection shows a subscription form at the bottom of each page.
Design your newsletter
In Settings → Newsletters, add your logo, choose the fonts and the footer. Send yourself a test before the first issue.
Write the welcome email
New members receive it right after signing up: say what they will get and how often.