Go to "My Workbench". Under the "My Websites" list is a "My Groups" list where you can edit some of your group's features. Select the site/group there.
1.) Once on your group's page, select the "Group" tab.
2.) Select the "Menus" link.
3.) "Add Menu"
4.) This is important -- Name your menu -- and it must be named "secondary-menu-xxxxx"
5.) Begin naming your menu items and offering node numbers; they will automatically appear under your main menu.