My understanding is that sub committees are still required the due process for meeting - in that agendas must be published, minutes taken, quorum etc etc.
If they are to meet behind closed doors, they would have to formally resolve to close it and not automatically assume that it is a private meeting.
With regards to a Councillors right to attend a correctly closed meeting, they don't have this right, I believe, as they are considered to be on par with members of the public and similarly they cannot vote or take part in the discussion. As an example, I don't believe that councillors not on the personnel committee are in a position to attend a personnel meeting if it is formally and correctly closed to the public - LTN 5 para 80
So, what is this subcommittee for?