Within the context of the original post, the point I think that was made is that a councillor cannot act without the authority of the council, so in the absence of a resolution of the council to authorise that councillor to act on the councils behalf the clerk ADVISES the councillor that he/she should not attend a working group with a third party, assuming of course that the reason for the councillor attending is to, on the face of it, represent the council. If the attendance at a meeting of the working party is solely as an individual, then I'd suggest that advice was not appropriate but then the councillor may have to consider declaring an interest in the subject if subsequently the business of that working party comes to the council for consideration.
S101 of the Local Government Act 1972 states that a council may delegate authority to "a committee, sub-committee or an officer of the authority" so delegation to an individual councillor does not fall within powers of the council.