How to register multiple member in one civicrm membership sign up form?
I don't think you can do this directly within CiviCRM via the user interface. I get around this problem by using the webform (https://www.drupal.org/project/webform) and webform_civicrm (https://www.drupal.org/project/webform_civicrm) modules. Using this you have a far more flexible approach, and can produce a sign-up form that can create memberships for multiple contacts.

I've never understood why CiviCRM, which has a membership management as a core function, does not provide this functionality itself.