First and foremost, the person has to apply to join the club. After you get the notification, go to Club, scroll down the club page and find the button that says Accept/Reject Applicants. Tap on it and it will take you to a page where you'll see everyone who has applied to the club and are still waiting for a response. From there you can Accept or Reject applicants. However, you won't be able to see them if the person has withdrawn the application or if another admin or the owner has already accepted or rejected them. Hope this helps!