。
She attended the meeting.
Literal
She [topic-は] that meeting [to-に] attended.
~に出席する is the formal verb for attending — meetings, ceremonies, classes, conferences. The に marks the event being attended; を would be wrong since 出席 takes に for the destination of one's presence. Casual speech would more often use ~に行った ('went to') or ~に参加した ('participated in'). 出席 is a Sino-Japanese compound (出 'go out' + 席 'seat') and lives in formal contexts: school attendance records (出席率 'attendance rate'), wedding RSVPs (御出席 'will attend'), and corporate meeting minutes.