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When I look at the newest users, I see some suspiciously similar names (user366, user350, user377).
Unlike other names though, when I type them into the "Type to find users" box, they don't appear in the results.
Those are the generated user names, just incrementing up from the first user-- everyone has a similar name, till they change it in their preferences under 'Display Name'.
In addition to what scw said, note that unregistered users won't show up in the user list.