To insert a new employee in the organization chart, click the Insert Shape button.

Organization Chart Insert Shape command - top level

At the top level of an organization chart you are given two options: Assistant or Subordinate. A subordinate would be an individual that reports to the person in the selected box. An assistant will come off the side and a subordinate will be below the box selected.

At other levels of an organization chart you will also be able to select Coworker.

Organization Chart Insert Shape menu

You can also insert employees by right-clicking any employee and choosing Subordinate, Coworker or Assistant from the shortcut menu.
If you have two workers that report to the same individual and are on the same level in the hierarchy, you can add them both as subordinates to their common supervisor, or select one of the employees and add the next employee as a coworker.