Each task in Asana can only have one assignee. This is intentional, so you always know exactly who is responsible for completing the work. In cases where multiple people are involved in the work product, you can create
subtasks
to track supplemental work—like reviews and approvals—connected to the main task.
Even though each task only has one assignee, you can add as many
task collaborators
as you want. Task collaborators are notified when a task is updated, which ensures the right stakeholders are looped in on who's doing what by when.