The National Council Session is considered a formal business event, and per GSUSA guidelines found on its uniform reference page:
Girl Scouts at each level have one required element (Tunic, Sash or Vest) for the display of official pins and awards which will be required when girls participate in ceremonies or officially represent the Girl Scout Movement.
For all girls, the unifying look includes wearing a choice of a tunic, vest, sash for displaying official pins and awards, combined with their own solid white shirts, khaki pants or skirts and a scarf. For adult members the unifying look of the uniform is a Girl Scout official scarf or tie for men, worn with the official membership pins, combined with their own navy blue business attire.