The tech company is a rapidly-growing business that doubles its staff every year.

rapidly-growing 是由副词 rapidly（快速地）和现在分词 growing（增长）组成的复合形容词。rapidly 修饰 growing，表示"快速增长"的状态。 rapidly-growing 整体作为形容词修饰 business，描述企业"快速发展的"特征。