He had to be dragged away from the party at Gibraltar Barracks, Camberley, Surrey, the court martial heard.
His alleged victim was at the party on April 4 last year to congratulate him, Bulford military court, Wilts, was told.
She said in court: “Things happened in slow motion. I was in total shock.”
Sgt Grundy denies any wrongdoing and sexual assault.
The trial continues.