The Charities Forum (originally named The Princes' Charities Forum) was founded by Princes William and Harry in 2006 as a way of bringing together their different charities and to foster collaboration and support between them. Each new patronage of William, Catherine or Harry is invited to join the Charities Forum. (Along with a select few charities, which the Royals are not Patron of).
The Charities Forum holds biannually meetings to allow William, Catherine and Harry to get updates on all their patronage's recent activities and projects, as well as allow the charities to learn from each other's successes and struggles. The Charities also develop relationships and collaborate on projects.
William, Catherine and Harry's patronages have worked together on many occasions over the years, as many have similar causes, and all a common goal of helping others.
Video of September 2009 Charities Forum Meeting: