The man on the left, Fred Astaire, was much better known as a dancer than a pianist, though I suppose if he wants to tickle the ivories, well, it's a free country. Meanwhile, the man in the middle, George Gershwin, was very well known as for his piano playing. He doesn't seem to mind Fred joining in (maybe he's giving him lessons.) The man on the right, George's lyricist brother Ira, is content just to watch. Since photographs are generally silent, I can't say what piece of music is emerging from that stringed instrument, but in 1937 the three of them (along with some blond chick named after a spice) came together for the film Shall We Dance:
They had three more films to go before they called the whole thing off.