The Wall Street Journal has the news.
A script is still in development and Warner has not set a release date, though the movie is unlikely to come out before 2018. Mr. Silverman would not comment on what other heroes might join Superman, Batman and Wonder Woman in the movie. However the studio has recently been casting the role of Cyborg, a half-robotic hero who is expected to have a cameo in “Batman vs. Superman” and then appear in “Justice League.” Other DC heroes who have been in Justice League comic books include Aquaman, Flash and Green Lantern.
According to The Hollywood Reporter the movie is being shot back-to-back with Batman vs Superman and will be released in 2017, not 2018 as the WSJ says:
Yes, Zack Snyder will direct Justice League. Yes it will shoot back to back. Likely release date: 2017. Likely announcement: Comic Con.
— Borys Kit (@Borys_Kit) April 28, 2014