https://www.law.cornell.edu/cfr/text/17/240.13d-5 seems to indicate they have to be explicitly coordinating to be legally defined as a "group" for this purpose (and Twitter's filing highlights this definition), which seems hard to prove. Twitter can't issue a search warrant, the participants would presumably not write any such deal down on paper, and the action is readily explicable by "I think the acquisition is gonna go through and he's offering a premium, so I'd profit".