Click the "My Forums" cube. See here how to add a cube to your home screen.
Click on the "Manage Forums" button.
Click on the "Add Forum"button.
Give the forum a name, a color and click on the "Add Participants" button.
Check the people you want to add and click the "Done" button.
Click the "Done" button.
The new forum has been created.
The friends you have added will receive a message that you have shared a forum with them.
If they press the "Open" button, they will go to the forum you shared. If the My Forums cube was not on their home screen before, it has now been added to their home screen automatically.