In Ghost version 3.0.0 A new "Member" feature is introduced. This feature enable the ability to accept your user subscription to your site.
Your members may be free or paid. To enable this member feature at first visit Lab page in your dashboard. There you will see an option "Enable members". Enable this option. After that you will see some option related to this option.
Fill all the details in those options and save the settings.
All members in your site are listed under "Members" page in your Ghost dashboard.
In this page you can see all your members. If you click on a member then you will see all the information about that member. You can see if that member is free subscriber or a paid subscriber. You can edit, remove or add an editor information here manually.