Here is the unpublished opinion. An excerpt:

The district court concluded next that joinder would not be feasible because the Tribe enjoys sovereign immunity as a federally recognized Indian tribe. Appellants challenge the validity of the Tribe’s federally recognized status but concede its existence. Indeed, the Tribe has been federally recognized since at least 1985, see Indian Tribal Entities Recognized and Eligible to Receive Services, 50 Fed. Reg. 6055-02 (Feb. 13, 1985), and it thus has “the immunities and privileges available to other federally acknowledged Indian tribes by virtue of their government-to-government relationship with the United States,” Indian Entities Recognized and Eligible to Receive Services from the Board of Indian Affairs, 77 Fed. Reg. 47,868-01 (Aug. 10, 2012).

Briefs and link to oral argument audio here.

Lower court materials here.