I think this would be fantastic. I have the same problem, and it often turns out that the search function does not actually return the user's posts, so I have to play a game of "guess the number" to try to figure out what page what user is on. I think the alphabet bar would be a welcome addition. Maybe it could be organized like a dictionary, for instance the first being Aa-An, or whatever it happens to be, and special characters at the end. Just a thought.