Superman and Batman are creatively the first two DC Heroes so using that logic:
Superman is the only super powered being on the planet KNOWN to mankind.
Batman is a masked vigilante in Gotham City but is a mystery as to whether the Batman really exists or is just an urban legend.
J'onn J'onzz is Detective John Jones metropolis PD- he has been on earth hiding for decades as the last of his kind until Superman arrives on earth. Sensing the great power of Superman he transferred to Metropolis police force to be closer to the Man of Steel- Initially to make sure he wasn't a minion of Darkseid(responsible for J'onzz race being eradicated).
Wonder Woman is on a Themyscara- an island of mythology that know one has ever been to because it lies in an area of the ocean that is a 'blind spot' to modern technology almost like our Bermuda Triangle phenomenon. J'onn J'onzz has sensed her existence and the existence of her sisters. They choose not to interfere in the events and lives of mankind.
Hal Jordan and John Stewart are Elite Eagle Squadron Pilots- the Green Lantern corps have not touched on them yet. The Green Lantern Corps is a Galactic Police Force in its infancy. This film in Act II sets the stage for the event where Hal Jordan gets the ring which would then be addressed in a Green Lantern film. He would join the corps and serve as Green Lantern until an event requires him in another sector of space which leads to his convincing the Lantern Corps to assign a ring and the policing of Earth to John Stewart. Jordan would serve as Green Lantern in Justice League I and Stewart would be Green Lantern in Justice League II
The Flash is a man named Barry Allen who resides in Central City. He is not the Flash just yet but he has his powers. J'onn J'onzz has sensed him also. The events of this film/story inspire him to don the red suit. Wally West exists but does not have super speed powers until...Justice League II.
J'onn J'onnz and Superman will recruit Wonder Woman and Flash while the Green Lantern will arrive for duty in Justice League I . We are working on getting devices in place which would have Hawkgirl asked by Hal Jordan to go to earth and help the League but we don't want it to be out of context or interfering with the plotlines or seem like she was added just to be in there like all the problems in 'X3 Last Stand'
Unfortunately as writers thinking of running times and budgets- these are the only choices currently for the series of scripts we have written. In order to serve a story better and create a remote possibilty that such stories could be used for big screen treatment. Unless its a bigscreen animated version the standard budget for a blockbuster summer movie is not enough for more than a team of five and we aren't writing Batman into any of the scripts out of respect for the beautiful world Chris Nolan created. It's a movie about super powered individuals coming together to defend against Darkseid and his Guard and the Batman that Nolan has created so wonderfully does not fit into this type of fantasy. It worked for the cartoon but even then Batman would always remind them that he was a loner and not the group type but if they needed him just call. So we leave Batman in Gotham fighting the mob and homicidal maniacs.