It might help to better understand what you are after. Are you trying to find out when the account was created? When they were granted permission to a particular piece of content? The first time they edited content? Something else?
I am sure there are multiple ways to do it. One is to look in CRXDE Lite (<serverURL>:<port>/crx/de). Users are stored here: /home/users. Find the user in question and then refer to jcr:created property. This will tell you when the account was created.