According to Wikipedia, we're either Parafamily or Ankle Biters.

What are Harry Styles fans called?

Harry Styles' fandom are called 'Stylers'.

What are Rihanna fans called?

The Navy. Rihanna's fans enlisted in the "Navy" following an infamous lyric from studio album, Rated R: “We're an army, better yet a navy, better yet, crazy, guns in the air.”