The theme of witness can be seen in several instances from chapter one. The most evident example would rest with the townspeople of Sighet. They are witnesses to Moshe the Beadle and the stories he tells about what he experiences. The witness the telling of his narrative and they reject what he has to say. In this example, the witness is an agent of denial, an act of negation. Moshe the Beadle, himself, embodies the theme of witness because of what he has seen. He witnesses Nazi atrocity after atrocity. Children being used as target practice. People forced to dig their own graves. Senseless killings that are preceded by the worst of deliberate cruelty. Moshe witnesses all of these and rushes back to Sighet to relay his experiences to others. For Moshe, the purpose of him being spared was for him to be a witness. It becomes even more stinging when this purpose is denied by the townspeople's rejection.
Finally, I think that one can see God as a witness. The relationship that Eliezer and Moshe share is predicated upon an exploration of God as a witness. When Eliezer asks Moshe about lacking the ability to understand God's answers and Moshe counters with suggesting that the questions are more important, God is a witness in Chapter 1. The witness position that God holds in Chapter 1 is a way to introduce the theme of God's role in the narrative, something furthered throughout the book.