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In this scenario you have to disable the groups that contain no items. I have attached a sample project that demonstrates how you can do this using the RadJumpList.GroupHeaderItemTap event.
Alternatively you can use a custom GroupPickerItemTemplateSelector to set a different template depending on whether the group item should be enabled or disabled. You can see how to do this in our Telerik Examples
application - JumpList - PhoneBook example. The default location of the source code of the application is C:\Program Files (x86)\Telerik\UI for Windows Phone 8 QX XXXX\Examples.
I hope this helps.
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