I just learned how to create a ButtonGroup using JFormDesigner, and it's pretty easy and pretty cool.
First, add all the related buttons you want on your JFormDesigner form. Next, click all the buttons while holding down the [Control] or [Apple] key, depending on your platform, so the all the buttons are selected at one time. Then right-click one of the buttons, then select the "Group Buttons" option from the popup menu.
That's all you have to do. Now, if you click the "Run" button (actually labeled "Test this form") you can click the buttons on your running form, and you'll see they now that they work as a group.