Unfortunately, GroupTable does not support groups without records, thus you cannot use it for your task. You can implement something like one-level tree with fixed one property grouping using TreeTable, but in this case you have to extract common base entity.
It can be done using two different techniques:
Use Single table inheritance for Group and Row items and extract base persistent entity that will contain all the properties shown in TreeTable. For group row that properties will return simply null values.
Use non-persistent entities for Group and Row and emulate Tree structure using CustomHierarchicalDatasource
If you use way #1 all the standard actions will work with your TreeTable, in case of #2 you will have more flexible solution, but you will have to implement standard Create / Edit / Remove actions manually.