If you only wish to update, create, deactivate or reactivate a single or a few users, you can do so in the Admin module via:
Admin → User Management → Departments and users → Users → Individual Update
A user must have a first name, last name, and email address in order to be created. We strongly suggest using an employee ID as well, but if your organization does not use them that's fine.
For information on adding users via CSV, please see our Adding & Updating Users Manually: CSV Upload article.