unlock article joomla after sign up + collecting mails

I need to insert simple sign-up form (pop-up box) with sign up buttons (email+password) when somebody want to read whole article... After registration/login he can read whole article. Articles are created in SP page builder in joomla. Sign up user can read all articles on web page - after they unlocked it... Administrator can choose which article is whole to read and which is locked.. Users should be written to database and easy export in csv from joomla. Also I need that it is written to database from what article (article link) user was sign up/login. person who "signed in" (registrovať sa) already doesnt need to sign in next time when visiting page that all pages are unlocked to him.