To be present at an event or procedure as a witness or attendant (e.g., at a birth, signing, inspection).
夫が出産に立ち会うことになった。
The husband will be present at the birth.
重要な契約の署名には弁護士が立ち会った。
A lawyer attended the signing of the important contract.
検査には必ず担当者が立ち会う必要がある。
A person in charge must always be present during the inspection.