This is the 746th most frequent Chinese word.
人群
Crowd.
In this sentence, '人群' refers to a group or crowd of people physically gathered in one place.
广场上聚集了一个人群。
A crowd gathered in the square.
In this context, '人群' denotes a characterized group of people, described here as 'complicated.'
面对这样复杂的人群，他感到有些不安。
Facing such a complex group of people, he felt a bit uneasy.
Here, '人群' represents the group of people among which an individual is distinguished.