Besides your registration form, there may be other forms that you need. You can create and manage those forms here. All your forms can be edited in the same way that you edited your registration form. See the Building Stage: Registration Form article to get an overview on how to edit forms.
To assign your created forms to a specific group, select the menu item marked "that belongs to any of the following group(s) that I choose:" and a set of groups will display that you can select from.