Apparently most of the crowd was unaware Hudson was in attendance sitting in a booth until Ferrell called her to the stage.
"Tonight I got to be in the presence of another one of our music greats, Rachelle Ferrell," Hudson tweeted. "A great reminder that real music still do exist."
Hudson can be seen in the upcoming "The Three Stooges" film, which hits theaters April 13.
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Sighting: Former NSYNC singer