What does Two-Bit mean when he says, "The only thing that keeps Darry from being a Soc is us?"

Because of his athletic ability, popularity, and looks, Darry would have been accepted by the Socs had he not chosen to recognize his family roots and associate with his friends and brothers.  Darry made the decision that his extended family (including the other boys) was more important than belonging to the "in crowd".  He threw away an athletic scholarship, which would have given him recognition and social standing via performance and education, in order to take care of his family.  Darry's priorities kept him from being a Soc.