This plugin currently allows you to create two different types of groups. Category and User groups, category groups are what allow you to create the hierarchy of groups. Category groups do not have members attached to them. When you visit a category group you will see two extra sub nav items, Sub Categories and Groups. The Sub Categories page displays all the category groups categorized under the group you are currently viewing. The Groups page displays all the groups (which are user created) that are also categorized under the group you’re currently viewing. Category Groups can only be created by the site admin, they can be created in wp-admin or you can choose to create a category group instead of a user group when you visit /groups/create.
More info and download: http://wordpress.org/extend/plugins/bp-group-categoriestypes/