On July 18 , 2007 in Johannesburg , South Africa , Nelson Mandela , his wife GraÃ a Machel , and Desmond Tutu convened a group of world leaders to contribute their wisdom , independent leadership and honesty to tackle some of the world 's toughest problems . On 18 July 2007 in Johannesburg , South Africa , Nelson Mandela , GraÃ a Machel , and Desmond Tutu convened a group of world leaders to contribute their wisdom , independent leadership and integrity to tackle some of the world 's toughest problems .