attendee; those present
Refers to people who have come to a meeting, gathering, or event. Slightly formal; common in announcements and reports.
The number of attendees exceeded expectations.
Materials were distributed to those present.
出席者 also means 'attendee' but is used for formal attendance, such as in classes, ceremonies, or official meetings. 来会者 is less about formal roll-call and more about the fact of coming to a gathering.
Compound of 来 (come), 会 (meeting), and 者 (person). The reading is on'yomi throughout.