Can I assign a task to multiple users?

A task can only be assigned to one user at a time.

You can, however, use checklists to create subtasks and assign them to various people.
Email notifications are also sent on checklist item assignments, just as they are for a task assignment. Individuals can also search all boards simultaneously for their assigned work - both tasks and checklist items - so there is no risk of anyone not finding items assigned to them.

To find all your assignments, just type @XY (an @ with user initials) into the search:

You can also turn to Task Dependencies, to show a hierarchy of related tasks, with each of them possible to assign to a different team member.