Hide custom field value

If the registered users are all within the WP user table then you can do something like this

<?php 
 if( is_user_logged_in() ) { 
  /// link to membership section
 } else {
  /// link to signup page?
 }
?>

This will check the current status of the user and see if they are logged in or not.

More info: https://developer.wordpress.org/reference/functions/is_user_logged_in/