The RadListView displayed the data that is bound to it, so if you need to add item manually (not from the control) you need to add this item to the data source and rebind the control. As for adding the item to particular group, note that those groups are created based on specific data field values, so if you need the new item to be in particular group, the value of the data field that is used for the grouping should be equal to the value of the group.
Finally, you can take a look at the following online demos and see if they could be of any help for your requirement:
Hope this helps.
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