noun
Refers to a group of people, often with a slightly negative or informal connotation, like a gang or a bunch. Can also be used neutrally for a party or set of people.
彼は悪い徒とつるんでいる。
He hangs out with a bad crowd.
徒を組んで行動する。
They act as a group.
仲間 is a more common and neutral word for a group of companions or colleagues, while 徒 often implies a less formal or slightly negative grouping.
連中 is a colloquial and sometimes derogatory term for a group of people, similar to 徒 but more explicitly informal.
The kanji 徒 originally means 'on foot' or 'follower', and by extension came to refer to a group of followers or a party. The exact historical derivation of the reading と is uncertain.