Currently the User Name field under User Criteria in Alert Users does not have auto-complete nor does it have comma-separation (unlike in, say, the User Criteria in Notices). This makes the feature in Alert Users very frustrating. If you have members named Mark, Mark5, Marks. And you specify Mark in the User Name field, it will send the alert to all those three even if you only want to send to Mark. I wish it worked the same way as in Notices.