I would certainly expect a working group to be set up by a committee or council in a properly constituted meeting and not for it to simply appear out of nowhere! They should have proper terms of reference and although they can be run informally, they cannot make decisions but can usually make recommendations for consideration.
Working groups are often set up for a specific project so in effect are task and finish groups with a finite life and limited brief. Used effectively, they can do the "leg work" on a specific project (gathering information for example) but in my opinion lack transparency although their lack of decision making powers limits their role.