Unfortunately, I cannot find a complete list of participants. The Royal Collection has this, "Group full-length photograph including Prince Albert Edward, Prince of Wales; Princess Beatrice of Edinburgh (Baby Bee); the Duchess of Edinburgh-Maria Alexandrovna with Princess Alexandra (Sandra), and Hereditary Grand Duke Ernest Louis of Hesse behind them; Prince Alfred of Edinburgh at the front with Princess Marie of Edinburgh -later Queen Marie of Romania; behind them are Princess Victoria Melita of Edinburgh; Prince George of Wales (later King George V), and Prince Alfred, Duke of Edinburgh.” A cropped version of this image is also on the Internet.
Two Ladies stand out, Maria Alexandrovna in a possibly brocade dress and her daughter Marie (“Missy”) in a light-colored dress. Victoria Melita is squeezed behind her brother to the right. All of the men wear uniforms - no civvies here.
From Wikimedia.