I too am making the assumption that the term "portfolio" actually refers to working groups or task and finish groups which are not uncommon. The main difference between a working group and a committee is one of transparency as working group meetings don't have to be held in public and nor are they required to publish agendas or minutes. However, they cannot make decisions on behalf of the council but can make recommendations. They can be useful in undertaking background work, for example, on a proposal but in my experience can go off topic a bit unless there are clear terms of reference. The important point is that they can never make decisions or commit expenditure and must report back to the body that set them up (whether a committee or full council) with their findings/recommendations which is where the public scrutiny can take place.