grassroots group; spontaneous assembly of ordinary people
Refers to a loosely organized, often informal group formed by ordinary citizens around a shared interest or cause, without top-down leadership. The term carries a nuance of self-starting, voluntary participation.
A grassroots group was formed to liven up the local festival.
They started cleaning up the park as a spontaneous group of volunteers.
任意団体 is a legal/formal term for a voluntary association. 勝手連 is much more casual and implies a loose, ad-hoc gathering rather than a formally registered group.
Compound of 勝手 (katte, 'one's own convenience; self-willed') and 連 (ren, 'group; party'). The term suggests a group formed at one's own initiative, without waiting for official organization.