There are differences of opinion about working groups. Some people argue that any group of councillors has to be a committee or sub-committee. If it is, then all the rules for agendas, minutes and so on will apply. There are also constraints on where a formal meeting can take place.
Others, including me, believe that there is value in having informal groups who can work on specific issues to explore options and make recommendations. It would be unlawful for such a group to make decisions, and in most cases it is highly desirable that the group's conclusions should be made public.
To give a concrete example, the council of which I am a member had an old building that needed to be upgraded and turned to new uses. An informal group of councillors, along with a local building designer, held detailed discussions (round one of the councillor's dining table) and hammered out a specification for the building work to be done. This was made public, approved by the council, and put out to tender.
My view is that this is the kind of thing that in a larger council would be carried out by officers in private, and then put to the council for final decisions to be made in public. So I see no reason why small councils should not do something similar through the voluntary efforts of councillors and any helpful local people.