。
She attended the gathering.
Literal
She [topic-は] that gathering [to-に] attended.
会 ('gathering, meeting, society') is a flexibly broad word — a 同窓会 is a class reunion, a 茶会 a tea ceremony, an 学会 a learned society. Without modifiers, plain 会 reads as something like 'the gathering' — context picks the specific kind. ~に出席する keeps to the formal register: the same polite 出席 used for business meetings, school attendance, and ceremonial presence. Its understated tone suits situations where the listener already knows which 会 is being referred to.