Postgres and Windows AD using windows authentication
First you have to note that PostgreSQL doesn't really have a concept of anonymous usage.  So you do not have a way to log in without the user being added.

You have two options. The first is you can add appropriate ad users and groups manually.  Or you can add them using an AD trigger (see https://superuser.com/questions/909554/trigger-an-event-when-computer-is-added-to-active-directory for a similar question, and full documentation at https://technet.microsoft.com/en-us/library/dd277403.aspx).  Please note, the latter is a bit of a security concern and you need to make sure you aren't giving out access beyond what you want to give).

If you go the latter direction, you need to make sure you are restricting access to the public role.