Let's say they go for five members - three are going to be Superman, Batman, Wonder Woman - how much exposition do those three need?
That's a good point. Man of Steel will be out next year - even if this JLA is set in a different continuity and doesn't have Henry Cavill (though I think it will), audiences will still know that he's the last son of Krypton, blah blah blah. Batman Begins will still be recent enough for most people to know why Batman does what he does, even though this incarnation will definitely not be in the same continuity.
WW may need a little more exposition but will she need any more individual time than say Storm in the first X-Men movie? And Black Widow and Hawkeye got a fair bit of screen time in Avengers without an awful lot of time being spent on either of their backgrounds (albeit that neither is a super-powered being).
Who else will be in this movie? Flash? Martin Manhunter? Green Lantern or Arrow? They can probably be summed up fairly quickly by either flashback or narrative.