Password protect all pages

Hi, I am trying to have my site be password protected and I am hoping for it to work like this.

Landing page is unprotected and can be accessed by anyone (but mostly just the people that are given the url and already have a password ready). From the landing page have a widget area that they can “Log-in” to the rest of the site, accessing all of the pages until they leave or log-out.

Is this possible? I know I can password protect individual pages but I would like to be so that the user can login once and access everything instead of entering a password for every page they visit.

Thanks for any help or guidance!