Use $_SERVER.LOGON_USER to set a session value?

On my IIS internal site I would like to use the $_SERVER.LOGON_USER value to set a session or permissions for the site so users don’t need to log on separately.

What is my best option for having that value available to set permissions on pages?

I’ve added a table to the database to connect the user logon with the permission I want for each how do I get that to generate a session value ?