Ordinary cooperation makes people useful to one another: a passenger relies on a driver and a patient relies on a clinician. The Kantian concern is treating someone merely as a means, subordinating their agency to a project without due regard for them as a person with ends of their own.
People routinely help one another achieve goals: a teacher assists learning, and a driver provides transport. Such cooperation is not automatically objectionable instrumental use. The concern becomes sharper when another person’s capacity to understand, choose, and pursue their own ends is bypassed or subordinated. Deception, coercion, and imposed sacrifice can therefore raise different questions from an openly agreed exchange. The important issue is how the person participates in the purpose their action or body serves.