Yes, Jamal Khashoggi Was a Member of the Muslim Brotherhood
Bin Laden joined the brotherhood at about the same time Khashoggi did, in the late 1970s. The two men shared a passion for the mujahideen's war in Afghanistan, first against the Soviet Union and later for power in Kabul. Khashoggi was covering the war as a journalist, but he was clearly sympathetic to the cause.
This argument is really about whether Khashoggi was a member of the Muslim Brotherhood or supporter of its ideals at the time of his death. There is no question he was a member in his youth and the association persisted for a very long time. Those who claim he was never a member and never espoused Islamist ideals are lying.