Only the primary account can add users.  To add a user: 
  • Select Settings and click Account Settings.
  • Next to Additional Users select the + New button.
  • Enter the name and email address for the user.
  • Save
An email will be sent to the added user for them to finish creating their account.

Note: If you do not see the Additional Users section then you are not the primary user for your organization.  Contact your primary user to make the changes.