Yesterday, IGN posted a new interview they had conducted with Dwayne "The Rock" Johnson. During their conversation Johnson was asked if he believed his Black Adam could join the Justice League. To which he replied, "I think so."
In the comics, Black Adam hasn't always been portrayed as a bad guy and for a short time he was a member of the JSA, but as far as I can remember he has never been a member of the Justice League.
Maybe someone explained that to Johnson as he has revised his comments via twitter. Instead of joining the Justice League he says Black Adam should fight them.
Justice League Part One will open in theaters on November 12th, 2017, followed by Justice League Part Two on June 14th, 2019.