This is the 2953rd most frequent Korean word.
참석하는
"Attending" or "participating."
Here, "참석하는" is used as an adjective to describe the act of attending in the present tense.
그는 회의에 참석하는 것을 좋아합니다.
He likes attending meetings.
In this case, "참석하는" is used as a present participle to describe the act of attending something scheduled for the future.
우리는 내일 세미나에 참석하는 계획이 있습니다.
We have plans to attend the seminar tomorrow.
The word "참석하는" acts as a modifier to "사람들" (people), specifying those who are attending.