[JTree] Adding a leaf to a node

Hi I'm working with JTree and I need to know
how to add to a specifique node a leaf.

My tree will represent a contact list and all the nodes are groups.
And I would like to add to a particular groups an user.

I began to try something like that :

public void addUsers (String userName , String group, String statut) {

for (int i = 0; i < rootNode.getChildCount();i++) {
if (rootNode.getChildAt(i).toString() == group) {
DefaultMutableTreeNode leaf = new DefaultMutableTreeNode(userName +
" " +
statut);

// And here I would like to add to the node at [i] a new leaf

}
}
}

Thanks for your help.

Alexandre Jaquet