It is composed of seventeen House Representatives or Delegates from each of the seventeen Houses, of which the Heirs of each House presumably have the highest seniority. There is likely oversight from the Lyorn and/or Issola as regards protocol.
The Council of Deputies come together at the Meeting of Principalities at an interval which is probably once per phase. Not all 289 (17²) delegates will necessarily come, but presumably a full quorum is not absolutely required. However, a full quorum is sufficiently desirable that only personal emergency is acceptable as an excuse for nonattendance.
A subset of this group is the Council of Princes.