This is an awesome features of Ghost. If you want to make money with publications, you can use Membership features of Ghost.
Go in your Ghost's
dashboard -> Settings -> Membership -> Portal Settings
By default, Simply uses Portal for member management. If you prefer, Simply also comes with individual pages to support the membership function.
If you want to use with individual pages let me know. I will create a more functional version.
Many people have been asking me to disable membership.
You will now be able to disable the login buttons and forms.
Dashboard -> Site design -> Site-wide -> Disable member
If you want to completely disable the membership.
Dashboard -> Membership -> Subscription access -> Nobody