The correct answer to this open question is the following.
The athletes that appeared with Muhammad Ali in Cleveland after his announcement to refuse to be drafted were Pro football runningback start Jim Brown, who played at the Cleveland Browns at that time; as well as other athletes like Bobby Mitchell, Jim Shorter, Bill Russell, Carl Stokes, Willie Davis, among others.
Here, we are talking about the famous meeting of June 4, 1967, in Cleveland, Ohio. In this city, renowned African American athletes met to express their support to Muhammad Ali's announcement to refuse to be drafted. The event was known as the Cleveland Meeting.