There was a time when the Justice League was an elite enough organization that a hero being offered membership was a big deal—and a hero turning down that offer, like Metamorpho or Black Lightning did—was huge. In the decades since, though, the team has been through enough roster shakeups that virtually every sort of superhero you can imagine has spent some time on the team, including some pretty weird entries.
The angel Zauriel, for instance, was a literal guardian angel who fell in love with the woman he was meant to be watching over, and descended from Heaven to be with her—and also to help the Justice League fight a war against rebellious angels who were trying to destroy the entirety of Creation. On the flip side, when the core team was trapped in the distant past, Nightwing formed a replacement Justice League that featured Etrigan the Demon. Or at least, it featured Jason Blood, Etrigan's human host, but let's be real here: saying Jason Blood's on the league and the Demon's not is kind of like saying that Clark Kent isn't on the team because Superman's the one who got the key. And Congorilla… well, he's a talking gorilla. That doesn't really require much more explanation, does it?
The biggest expansion of the roster, however, came in JLA #41. When Earth was being attacked by Mageddon the Anti-Sun, a primordial annihilation machine that had journeyed to Earth from the far reaches of the galaxy, the last line of defense came when alien technology allowed the League to give every person on Earth super-powers. They all rose up to help Superman, who had been trapped my Mageddon, leading Wonder Woman to refer to them as the "Justice League reserves."