Cliques inevitably group people in terms of what they have in common, but not everyone likes emphasizing the common. The critical theorists, Adorno among them, argued that society put too much focus on everyone being the same.
Society pretty much says that you must belong to a clique because, you know, that's your "identity." Adorno and the gang wanted you to know that you don't need to belong to a clique. It's probably better if you don't.
Other members of this group included Max Horkheimer, the founder, who first began the Emancipation from Cliques movement; Herbert Marcuse, who called you one-dimensional if you aspired to join a clique; Walter Benjamin, with his politically-oriented theory about cliques in literature; and Jürgen Habermas, who sought to reform the Anti-Clique Clique in response to the social development of other cliques in his day.