Until 2014, the style council was the institution in MusicBrainz that created and edited the official style guidelines, by following the the proposal system. The style council has been reformed many times, as it did not work well initially. See the history of the council for details. In October, 2014 it was replaced with a Style Benevolent Dictator1.
There was no formal membership to the style council. If you were an interested contributor to MusicBrainz and reasonably well informed about existing style guidelines, MusicBrainz development and the general culture of the project, you were welcome to subscribe to the style mailing list and join the discussions.