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 "Quick add and invite".
Enter your employees' names and email addresses from the screen (see screenshot below)
If you select "Send invite immediately" each employee will be receive an email with login instructions immediately. If this option is unchecked the employees will be added to Air but you'll need to invite them one-by-one.
Click "Add employees" to add people.