I have watched Smallville as far as Season Six, Episode Eight. Of the original Justice League of America membership, four have so far appeared:
the Martian Manhunter.
These three have all been dramatized but not in this continuity yet. They are also major figures. However, it seems that Green Arrow and Cyborg, who have also been introduced, will be charter JLA members in this continuity. Looking ahead, GA, Aquaman, Cyborg and Impulse will ask Clark to help them against Luthor in Episode Eleven called "Justice."