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". People assisting or helping the rapporteur are called shadow rapporteurs and may vary in number depending on the issue.
See also 
(References:) Oxford Online Dictionary