Adding Multiple Locations for a Contact in the Web Portal can be done from the Contacts List view, the Locations List view, or from the full Contact page view as well as from the new job page.
To add a new Location via the Contacts or Locations list views through the Web Portal > CRM > All Contacts > choose either the Contacts or Locations list > Click on the Contact/Billing Name > select Location > + symbol > add the new location details and Save.
To change it via the full contact page, CRM > All Contacts > Select the Contact ID or Arrow in the list > Locations + Symbol > Add Location > Save.

To add multiple location on the App > CRM > Select the client you would like to add the Location to > Locations > + symbol at the bottom right.
