▲表示“在聚会上”“在晚会上”等，通常用介词 at。更多例句如：

Who else was at the party? 聚会上还有谁?

He enjoyed himself at the party. 他在聚会上非常愉快。

Were there many people at the party? 聚会上的人多吗?

He finds it hard to mix at parties. 他感到很难在聚会上与人攀谈。

Guess who I saw at the party yesterday? 你猜我昨天在聚会上遇见谁了?

I am glad that you had a good time at the party. 我真高兴你在晚会上玩得开心。

▲但《英语常用词用法详解大词典》认为用 in the party 是错的，也有点过于绝对。其实用 in the party 也是可能的，只是不如用 at the party 普通而已。如以下各例句均摘自词典或语法著作：

The mood in the party was joyful. 聚会上的气氛是令人愉快的。（《朗文英语联想活用词典》）

John will sing in the party and Mary won’t. 约翰会在晚会上唱歌，但玛丽不会唱。（章振邦《新编英语语法》）

Each girl in the party was paired off with a boy. 晚会上每个女孩都与一个男孩配成一对。（张道真《现代英语用法词典》）

▲顺便补充一句：用 in the party 表示“在党内”，则是很普通的用法。如：

She was active as a left-winger in the party. 她是党内积极的左翼分子。

The accusations caused a split in the party. 这些指控使党内出现分裂。 

There have been deep divisions in the party over Europe. 该党内对欧盟问题存在严重分歧。

He had the unhappy knack of making enemies in the party. 他总是在党内树敌，弄得很不愉快。

He had manoeuvred his way into a position of strength in the party. 他设法在党内获得了实权地位。