In my day your coach would fill in a card to show you'd passed Skate Star and I think you would send that off to NISA with an application for membership, but that was more than 10 years ago so things might have changed. Your coach should be able to explain the process. I assume you want to join NISA because you intend to compete and/or take formal tests? There's nothing to stop you carrying on with lessons and just getting assessed by your coach, if that's all you want to do.