Well in the first film we were to believe the League pretty much controlled the city and had significant enough influence over it to force the city into a depression. I don't think the idea that Catwoman and Bane are members as well is a big leap once you sell the audience on their influence and reach, at that point you could very easily say that two major members are also operating in Gotham. I think the stretch would be if they tried to retroactively say the Joker was a former member or something, which they won't. Just saying I don't think the idea is all that out there.