Simplify the following text for a Grade 9 native English-speaking person.
Original: 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 together to the world .
Simplified: On 18 July 2007 in Johannesburg , South Africa , Nelson Mandela , GraÃ a Machel , and Desmond Tutu began a group of world leaders to help the world .