First, Elie Wiesel wrote that "for the living and the dead, we must bear witness." Therefore, someone who witnesses is someone who tells the story of those who lived and died in the camps. Elie Wiesel is a witness who tells the story of all those he encounters at the camp.
In chapter one, Moishe the Beadle is a witness. He is expelled and sent on a train, but when he returns, he warns the Jews of Sighet of the horrors that await for them at the hands of the Nazis. He tells of those who were shot and killed in pits, and he asks those in Sighet to leave, to run and to save themselves. No one wants to believe that horror. Moishe the Beadle tries to save the Jews and tells the story of those who died.
I consider this to be a portrait of Moishe the Beadle. Elie Wiesel, the author, details the wisdom and strength of this man before and after the horrors he saw. Elie recognizes why the Jews wouldn't listen but acknowledges Moishe the Beadle's courage for coming back to warn them anyway.
Therefore, by Wiesel's definition, Moishe the Beadle is a witness to those taken, and Elie Wiesel is a witness for this portrayal and detailed account of Moishe the Beadle's attempted heroism.