Tracking/Recording all users that log in especially Active Directory users

Hello, I believe this is my first post here. I've been trying to get a script to work that uses "whoami" and sends the results to a text file. The script works and writes to the file, but when an AD person logs in it seems spotty and usually records root instead of the actual user. Does anyone know of a log file that alreadys records all of this or any ideas of how best to record this information? Thanks David