Go to your dashboard and click the "Add" circle in the People section. Alternatively, go to the People app and click the "Add employee" button.
Select "Add employee with details".
Enter the required information then click "Save and continue" to move on the next step. Repeat for the next step(s).
At the end you can choose to invite the employee now or later. When you invite them they'll be sent an email with instructions to login and complete their profile.