The Ultras are a clique of popular girls led by Chris Hargensen. They are the main antagonists of the film. All members, except Sue, who was not an antagonist, were killed on the night of June 22, 2013, prom night at the hands of Carrie White.
It seems that all of the Ultras except for Sue, HeatherAnnie Chung and Red Dress Girl were involved in the prank. Heather was originally aware of the prank, however, Samantha Weinstein believes that she wasn't in the final cut. Nicki and Lizzy are never confirmed to have been a part of the prank, however, they seem to be aware that something is coming.