There were a lot of people at her brithday party . She had met only a few of these people before ( MDQH) There were a lot of people…

There were a lot of people at her birthday party, only a few of whom she had met before

Ở đây ta thấy A few of these people chính là a lot of people ở câu trước ⇒ Dùng whom để thay thế