Conventional wisdom says that Yehuda suggested that the brothers sell Yosef, the brothers accepted the suggestion and subsequently Yosef was sold. When he reveals himself to his brothers, Yosef says to them, "I am Yosef who you sold to Egypt!" Therefore, it appears that the answer to our question is Yosef's brothers. This is the position of the Rashi.
However, a closer reading of the story, combined with the geographical details of the story, make it feasible that the brothers did not sell Yosef, as is the position of Rashbam, Rashi's grandson. If that is true, the brothers suggested killing Yosef, leaving him to die in a pit and selling him but never actually went through with any of these suggestions.
