网友的问题，涉及“主谓一致”。

an army of （一大群）作定语修饰名词时，跟a group of  用法一致，谓语动词用单、复数均可。用单数时，意味着中心词是army；用复数时，意味着中心词是后面的复数名词。

例如：

An army of workmen was brought in to build the stadium.  

一大群工人被召来建筑运动场。

来自《简明英汉词典》

An army of bees were flying around.  

一群蜜蜂到处飞着。

来自辞典例句