This is a short expression that is apparently the best that can be found; it is as good as an adjective.
(merriam-Webster) in attendance
1: present at an event, meeting, etc.
- Everyone in attendance voted in favor of the measure.
- A number of celebrities were in attendance.
- One of the employees in attendance asked a question.
(ref.) The gym was over half full (more than thousand people in attendance) and most of the people in attendance were white.
(ref.) The meeting started at 7:30 p.m. with five people in attendance
(ref.) The total number of people in attendance at the funeral home confirmed that the news of Mrs. Elberhart's death hadn't attracted much attention
refs
However, this acception of "in attendance" could be particular to AmE. For instance, the Cambridge dictionary does not mention the sense found in the Merriam-Webster dictionary, nor does Collins.