Password protect entire site – with multiple users

I’ve been asked to design a site as a staff intranet and I’m wanting to use WordPress. However, as the site will be accessed by over 100 employees and, because it contains sensitive information, I need to be able to assign a unique username and password to every employee (that way if they leave the company we can revoke their access).

Is this possible? Does it involve a (or several) plugins?

Thanks in advance