Peter(Pjotr) Ewlampieff (sitting under Sergei Rachmaninoff and his wife Natalja Alexandrowna Satina), Wassily Flustikoff, second from the right and Peter Kowalewsky, first from the right.
The man in the suit next to Rachmaninoff could be Jacob Mulmann
But I’m very unsure. It could also be Rachmaninoff’s cousin V.A.Satin (his wife Natalja’s brother), because the two look similar.

In the years from around 1927 until the choir left for the USA (april 17,1939), Jacob Mulmann, a Russian teacher and director of the library in Dresden founded in 1908 after A.I. Chuprov, worked as the choir’s secretary. He also received a small salary from the Don Cossack Choir.
As a passionate lover of the Russian art, he also assisted other visiting Russian artists such as Rachmaninoff.
As it turned out, Jakob Mulman was the person to whom the Don Cossack Choir owed a lot.
Mulmann was responsible for the preparation and printing of songbooks and textbooks that were published by the Don Cossack Choir publishing house, dealt with the numerous correspondence with the concert management, communicated with lawyers and participated in the preparation of program documents and advertising posters.