Halliday & Matthiessen (1999: 192):
institutiontypifying roles: active participant in figure of saying [Sayer, e.g. the ministry announced...], of doing [Actor, e.g. the school is closing down]; also of sensing, typically thinking and intending [Senser, e.g. the class decided that...].pronoun it ~ they; general noun people, place, set-up etc.; number category: count (singular).²has potential for voluntary action, typically semiotic with authority of a collective [verbal process of ordering, mental process of deciding or judging]; also material [material processes, middle and effective].
² Institutions of course do appear in the plural — although relatively infrequently: the plural pronoun they typically refers to a single institution.