at a meeting和in a meeting都可以说，前者侧重表示在会上出席会议，后者则侧重表示在开会。如以下各例均引自权威词典：
Officially, the director is in a meeting, though actually hes playing golf. 据说主任正在开会，可他实际上正打高尔夫球呢。见《牛津高阶英汉双解词典》(第四版) (officially词条)
Im tied up in a meeting until 3 pm. 我开会要开到下午三点，此后方可脱身。见《牛津高阶英汉双解词典》(第四版) (tie词条)
Sorry to keep you waitingI got stuck in a meeting. 对不起，让你等了，我刚才在开会。见《朗文当代英语词典》(1995年版) (keep词条)
Mrs Lavelle is in a meeting at the moment. 莱维尔夫人此刻正在开会。见《朗文当代英语词典》(1995年版) (meeting词条)