We know that Luthor will be in Arkham Asylum (another deleted scene reveal) by the time Justice League begins, so perhaps Batman pays him a little visit to figure out what all that bell talk was about.
As for what Luthor could be doing in this film, his role could be as small as appearing in one scene to as big as being a secondary antagonist.
That’s assuming that he will be an antagonist, though.
While the motivations of Eisenberg’s Lex Luthor are pretty difficult to track, traditionally Luthor is on the side of humanity, which means he won’t want the planet taken over by aliens any more than Superman (hating aliens is kinda Luthor’s thing).