A rapporteur is a person who is appointed by an organization to report on the proceedings of its meetings.
For example, Dick Marty was appointed rapporteur by the Parliamentary Assembly of the Council of Europe to investigate extraordinary rendition by the CIA.
Authors of reports prepared by committees of the European Parliament are also known as "rapporteurs". Authors of reports assisting a rapporteur in preparation of committees of the European Parliament are known as shadow rapporteurs, and may vary in numbers depending on the issue.
(References:) Oxford Online Dictionary