The lady sitting on Ellington's right is Miss Barbara Reid. She was also a pianist and worked for the Stratford Shakepearean Festival in some capacity as a publicist. Clearly, she knew Ellington and liaised with his management prior to his several appearances at the Festival. About twelve months ago, a scrapbook of Miss Reid's cuttings came up for auction on Ebay. Here are two photographs from the volume.
Bill Mark, Photo of (left to right) Mary Jolliffe, Michael Langham, Barbara Reid and Duke Ellington at the Sheraton Park Hotel in New York, April 1957.