Unite the how many now? Late Thursday night, director Zack Snyder released the first image of Jason Mamoa as Aquaman in Batman v. Superman: Dawn of Justice. The image was overlayed with the message "Unite the Seven," without any explanation of what that means.
Could it be a reference to the "big seven" of the Justice League? Maybe not. According to BadAss Digest, only six members of the League were planned to appear in Batman v. Superman, according to the last script they peeked at, though there's obviously still plenty of time for changes and rehoots:
About "Unite the Seven" - it's worth noting that script with which I'm familiar had Superman, Batman, Wonder Woman, The Flash, Aquaman and Cyborg. There was no seventh in the script, although it's possible that the seventh member of the team - Green Lantern, one assumes - is introduced in Justice League. I had heard that Lantern wasn't showing up until the very end of Justice League Part 1, but maybe that's changed. And who knows - he could still show up in BvS. The scripted appearances by Aquaman and The Flash are very, very brief - cameos, really - and it would be easy to add Green Lantern in, possibly even with reshoots closer to release.
Personally, I had assumed that "Unite the Seven" was a reference to the Seven Seas, being that Aquaman is the "King of the Seven Seas," and Snyder's tweet specifically refers to Aquaman as the "one true King."
Another possibility is that it could refer to the Others, Aquaman's non-Justice League superhero team, which also consists of seven members. This idea might clash with Devin Faraci's assertion that Aquaman's appearance will be cameo length, but we've seen hints of at least one member of the Others, Kahina, on set.
Batman v. Superman: Dawn of Justice opens March 25, 2016.