3 manning all Challenge Modes on GOLD! - Youtube.com/MyCelar
They are registered at wowprogress and confirmed their characters, I think.
Those appear to be toon names and PvE scores as shown on wowprogress.
For my guild, the only people who have them are the registered guild administrators.
So either the 'u' is there to indicate that person is a registered user of wowprogress, or that they have privileges to edit their guild setting and update their current progress more quickly that the average site visitor.
If I were to guess, I'd say that registering with the site, and verifying/claiming you character would garner the 'u', to show that it belongs to a site user.