Membership
Neutrino gives you two ways for readers to sign up and sign in, and you can use either or both. Ghost's own Portal pop-up works out of the box with no configuration at all — a right-hand overlay that handles signup, login and account management. Alongside it, Neutrino ships a pair of full-page signup and sign-in screens, designed as part of the theme rather than as an overlay, for sites where becoming a member is a considered decision rather than an afterthought.

Beyond the signup surfaces themselves, two parts of the layout notice whether the reader is signed in. An Account link appears in the header for members and is absent for everyone else. And the footer newsletter form removes itself for signed-in readers, on the reasoning that showing a subscribe form to somebody who already subscribed is noise — with the footer re-flowing to fill the space rather than leaving a gap where it was.
All of it depends on Ghost's membership system being switched on. That is one setting, and if signup appears broken everywhere on your site it is overwhelmingly the first thing to check.