What council was Joseph a member of? None other than the Sanhedrin, the 71 "elders, men of age and experience, and scribes, lawyers, or those learned in the Jewish law" that made up "the supreme council of the Jewish people" (Smith's Bible Dictionary). This same group condemned Christ, yet Luke calls Joseph "a good and just man." Herbert Lockyer's All the Men of the Bible, p.204, comments:
As the Bible never uses words unnecessarily, there must be a distinction between "good" and "just." As a "good man" we have his own internal disposition—what he was in himself. As a "just man" we have his external conduct—what he was towards others. His just dealings were the fruit of the root of his goodness. His was the belief that knew how to behave.
Luke also informs us that he had "not consented to their [the Sanhedrin's] counsel and deed." This could mean that he had simply not voted with the majority, or it could mean that he had gone so far as to speak out against their actions. The Interpreter's Bible suggests, "Whether he merely withheld his vote or actively disagreed with his colleagues in the determination to be rid of the troublesome prophet from Nazareth is not clearly stated" (vol. 8, p. 414). In their commentary, Jamieson, Faussett, and Brown feel that "he had gone the length, perhaps, of dissenting and protesting in open council against the condemnation of our Lord." The Ambassador College Correspondence Course (1986 ed., Lesson 25, p. 11) concludes that Joseph may not even have attended the Sanhedrin's trial of Jesus. Luke's wording could be taken to mean that he disagreed by not attending, or that he was there under protest yet did not vote (see Mark 14:64: "they all condemned Him").
Joseph of Arimathea