It was hard to tell who was a guest and who was part of the act when “Natasha, Pierre & The Great Comet of 1812” — a self-described “electro-pop opera” based on “War and Peace” — gave a 45-minute VIP preview at Soho House last week. On the club’s sixth floor Tuesday night, a band began to play when some members of the snazzily dressed crowd (who seemed like spectators) suddenly joined in, performing scenes from the musical while also somehow throwing back vodka shots, jamming on instruments and jumping onto the backs of couches and chairs. Dave Malloy’s creation had an acclaimed off-Broadway run last fall and will move to a temporary structure between the High Line and the Standard Hotel starting May 1. The pop-up space, dubbed Kazino, was hatched by producers Howard and Janet Kagan by teaming with “Sleep No More” whiz Randy Weiner and Simon Hammerstein of The Box. Till Sept. 1, Kazino will host “Pierre” performances as well as a full menu and bar and pre-and post-show live entertainment. “It’s kind of like putting on a wedding every day,” Howard Kagan joked during a private tour of the space.