I'm writing in my capacity as secretary of the Social Club of the language school to complain about the services provided to us during our visit to the Science and Technology museum.

capacity 在这里指"身份、职位"，源自拉丁语 capacitas（容纳能力）。在正式场合中常用来表示某人以某种特定身份或职位行事时的资格或权限。