Dr. Tarver, from the Michigan hearing I think, has just joined as a plaintiff. I would think that every legal voter who voted in this election would have standing. But I'm not a lawyer, so...

FWIW, the lawsuit itself states:

There is no Eleventh Amendment bar to the inter- vention by a private party whose rights are affected by an original action between States. See, Maryland v. Louisiana, 451 U.S. 725, 745, fn. 21 (1981) (“[I]t is not unusual to permit intervention of private parties in original actions”); Arizona v. California, 460 U.S. 605, 614 ([O]ur judicial power over the controversy is not enlarged by granting leave to intervene, and the States’ sovereign immunity, protected by the Eleventh Amendment, is not compromised”