I think that Hamid is able to develop the side characters of the novel as outsiders primarily because like Changez they, too, are unable to understand anyone emotionally. Characters such as Jim, Erica, and the American are distanced from being able to emotionally interact with Changez. In this, they, too, are outsiders because of the distance with which they must live. For example, Jim is probably the one person at Underwood- Samson that could understand Changez. He is the one person who would escape Changez's blanket condemnations. Yet, he is unable to emotionally connect to Changez, seeking to use work as the answer that can address all. For Changez, this is not suitable. It is for this reason that Jim remains as an outsider to Changez's world.
Erica is an outsider because she can only emotionally connect with a nostalgic past that no longer exists. Her love of Chris and a past that is no longer present is where she is an outsider. She is unable to connect to Changez. For his part, Changez is unable to connect to her emotionally because of the weight of her past. In this, Erica is shown to be an outsider because of her flight from the present and her seeking refuge in the past.
The American is an outsider because there seems to be some type of division that enables him to fully immerse himself in Changez's narrative. It is not fully clear as to why the American is there. Whether he is there for purposes to understand Changez or purposes to remove him, it is unclear. He listens to what Changez is saying, but there is nothing in terms of full understanding or full immersion in what is being said. To this extent, he is an outsider, someone relegated to the margins of the narrative. He listens, but cannot fully embrace what is being said. In this, the American is constructed to be an outsider, similar to other side characters in the novel.