The groups, assembled under an umbrella known as the High Negotiations Committee, said in a statement Friday that their participation comes in response to "sincere" international efforts to end Syria's five-year civil war.
The HNC says its team in Geneva will press for a transitional governing body with full executive powers and a pluralist regime in which President Bashar Assad and his associates will have no role. The statement also insisted on Syria's unity and the restructuring of the country's security agencies.
Syria's civil war has killed 250,000 people and displaced millions.