CONF...
Here is a link to ICAO Annex 13. If you look at Chapter 5, specifically sections 5-3 to 5-5, you'll find the criteria for who can be, who should be, and who must be part of the investigation, including who can have access to all or part of the investigation material.
http://www.airsafety.com.au/trinvbil/C619icao.pdf