Grouping is done by introducing additional <graph> tags: For each group/folder node <node> tag, there should be a <graph> child tag that contains the nodes that belong to the group/folder.
Unfortunately, no, we cannot provide you with a sample stylesheet, because we do not have any such stylesheets.
My suggestion is to create one or more simple grouped graphs in yEd and save these graphs in GraphML format. Then inspect the generated GraphML and adjust your stylesheet accordingly.