presented at the party 是分词作后置定语,由于不是从句,因此不能加从句引导词.但此句可改成从句,加 that（不能加 who,因为被修饰的名词中有 all）后再加 were,即可改成定语从句.
全句：All the people that were presented at the party were her supporters.
全句：参加聚会的人全都是她的支持者.