That is Olga Alexandrovna and Nikolai Kulikovsky in the background, long before their marriage. This photo was taken around 1908 or so. At the time, OA and Kulikovsky were "involved" although she was still married to Peter of Oldenburg. That is NOT Peter of Oldenburg in the photo.
The general belief is that this photo was taken by Nathalie's husband, Vladimir Wulfert, on one of the many outings the five used to go on. At the time, Wulfert was either unaware or ignoring the fact that MA and his wife were becoming closer.