Colloquial. 聚头 carries the warmth of an in-person reunion - friends, family, schoolmates. For a formal meeting you'd say 聚集 or 聚会; for a business gathering, 开会.
memory
Literally 'gather + heads': picture people leaning their heads in around a table. The Cantonese cousin 聚頭 (jeoi6 tau4) is also common - same image.
