If you want a true portal where the details are updated automatically for each client then you'll probably want to invest in a third party business management tool, like Dubsado and use that alongside your Squarespace website. Dubsado allows you to build client portals where clients can access project details, view documents, pay invoices.
Third party 'member' add-ons can't fully secure any data that is stored on Squarespace so aren't ideal for this purpose.
