Councils are made up of interest groups of the population. There are communal councils, work councils of coops, housing councils, garden councils, consumer councils, etc.
Attendance is not mandatory, put highly encouragen through the social structure surrounding these councils. My worker's council cohorts are my friends, my goworkers, etc.
Big decisions are decided on via federation. E.g. every communal council sends of delegates to regional council, which sends delegates to a national council, and so on.
The difference to parliamentary representation is the type of delegation: Representatives have what you call a "free mandate": they only are subject to their own conscience (and the law, of course). If I vote for a representative for their strong stance against puppy butchering, they're free to butcher as many puppies as they like, once they are elected.
Compare that with an imperative mandate (which social anarchists propose): Your position as a delegate depends on you carrying out the will of the body that elected you. If you defy that imperative, you lose your position as a delegate.