Well, there's a crappy user search here. Or you can wait until AGv3 where a user search (a much better one) will be implemented.
Also, if you know their exact username, type in armorgames.com/user/blahblahblah. Replace the "blahs" with the user's name, and press enter. It should direct you to the user's page. If you get a 404 error, it's either the user has gotten banned, or it's not a real user (check your spelling as well).
