Give an example of the negative impact that co-consumers can have.

Examples may vary.
Other patrons or co-consumers can have negative impacts. For example, if another consumer touches a product in the retail context, this can lead consumers to negatively evaluate the product, an effect referred to as consumer contamination.
The presence of large numbers of people in a consumer environment increases arousal levels so that a consumer's subjective experience of a setting tends to be more intense. Although the presence of other people creates a state of arousal, the consumer's actual experience depends on his or her interpretation of this arousal. The psychological state of crowding exists only if a negative affective state occurs as a result of this density.
In addition, the type of consumers who patronize a store or service can serve as an attribute. To paraphrase Groucho Marx, "I would never join a club that would have me for a member."