I'm not familiar with this field; today I'm afraid I'm just here to make up the numbers.
Tips
history
From 《韩非子》: King Xuan of Qi loved hearing 300 musicians play the yu (a bamboo pipe) at once. A fraud named Nan Guo joined and faked playing for years. When the new king King Min insisted on hearing each musician individually, Nan Guo fled.
usage
Often used self-deprecatingly to humbly downplay one's qualifications in a group ('I'm just here to fill a seat'). When applied to someone else, it accuses them of being unqualified.