Pass oneself off as one of the Yu players in an ensemble 　 Be there just to make up the number.

During the Warring States Period (475-221BC), the King Xuān of the Qi State was very fond of listening to yu (a traditional musical instrument) ensembles and often got together 300 yu players to form a grand line-up striking up deafening music.

A hermit named Mr. Nanguo who couldn't play the yu, had an audience with King Xuan and boasted, then asked to take part in the ensemble band. King Xuan gladly consented and gave him the same reward as the others. Whenever the band played for the king, Nanguo just stood in the line-up and pretended to play. Nobody realized he was making no sound at all. So, he got his meals just as the other musicians did.

Later, King Xuan died and King Min succeeded to the throne. Unlike his father, King Min liked solos one by one. Mr. Nanguo became more and more frightened as his turn approached. One dark night before it was his turn to do a solo, Nanguo sneaked out of the palace and never returned.

The idiom sums up this fable. It is used to mock those who haven't real ability and learning make the number among the experts, or the inferior goods passed off as excellent goods. This set phrase show contempt when used to describe others and modesty when used to describe oneself.