From Resources > Users > External Users (tab) click + New

Enter mandatory fields:
Alias (Also known as Resource Code*Only required if not system generated)
Name, Surname, Title
Email 
Profile (*Profiles available when created based on Standard Access - External or Restricted Access.  For more information on Profiles click here)

Add optional information where applicable:
Customer 
Organisational Unit *Will only appear if Admin > Configuration > Company Preferences has "Activate Organizational Units" set to Yes

Hit the button OK to create the External User.