The original founding members of the Justice League were Superman, Batman, Wonder Woman, The Flash, Aquaman, Green Lantern, and Martian Manhunter (the last of which is replaced by Cyborg in the New 52 origin), the majority of which are present in Justice League (directed by Zack Snyder). By the end of the movie, one member is still missing, despite fans having expected him to turn up at some point. After all, the initial tagline associated with the film's marketing campaign was "Unite the Seven," and we only have six members.
The absentee founder is Hal Jordan, the Green Lantern. Fans everywhere got pretty excited after the SDCC trailer teased the existence of the Green Lantern Corps in the DCEU with Steppenwolf's, "No Lanterns" line. The film itself was obviously quite conservative with its depiction of them, which is understandable since no one has been cast as a Lantern yet.
So he doesn't make a full appearance, but his existence is teased in the film by a flashback scene narrated by Diana (played by Gal Gadot), when she lectures Batman about the last time Steppenwolf, the film's antagonist, threatened Earth. We can see that one of the warriors fighting is quite clearly a member of the Green Lantern Corp, though their identity is never shown, just their constructs and their ring, which flies off past Steppenwolf after the Lantern falls in battle, presumably to search for the next living being worthy enough to become a Green Lantern. Keep in mind that the flashback was depicting events that took place five thousand years ago so it goes without saying that the chances of that guy or his successor being Hal Jordan are pretty slim.
Still, the last character tease we had like this was in Batman v Superman: Dawn of Justice. In that film, Snyder teased audiences with The Flash, Aquaman, and Cyborg through one scene in which Diana sifts through videos of metahumans. It wasn't much and it wasn't immediately obvious when they'd be joining forces, but their existences were established and it got fans excited enough for the Justice League film that would follow.
Is the two-minute flashback sequence a subtle way of hinting at us that we'll be seeing Green Lantern added to the Justice League roster in the next film? Possibly. He's a founding member in both League origins and a pretty important one at that if the DCEU ever introduces Darkseid, the self-made God from the hellish planet, Apokolips. Green Lantern could be the one to introduce the more cosmic aspects of the DCEU to the films.
The inclusion of Green Lantern would also be in keeping with one of the film's motifs: the age of heroes. The first mention of the age of heroes is during that flashback scene with gods and lanterns fighting side by side. By the end of the film, Diana and Bruce go back to that notion, declaring that the age of heroes has returned. That would be abundantly clear if Green Lantern showed up to complete the set: gods, warriors, and other-worldly beings - Superman doesn't count as other-worldly, because he's more human than Bruce is (at least according to Bruce).
We know that a Green Lantern Corps film is in the works and that it will focus on both Hal Jordan and John Stewart, but which one of them will make it into the newly established Justice League, if any? A lot of it is up in the air. For now, we just have that one great flashback scene to gnaw on while we wait for answers.
Justice League stars Ben Affleck as Batman, Gal Gadot as Wonder Woman, Henry Cavill as Superman, Ezra Miller as The Flash, Ray Fisher as Cyborg, Jason Mamoa as Aquaman, Ciarán Hinds as Steppenwolf, Amy Adams as Lois Lane, Jeremy Irons as Alfred Pennyworth, Diane Lane as Martha Kent, J.K Simmons as Commissioner Gordon and Jesse Eisenberg as Lex Luthor.