A radio group contains a set of cells of radio types. Use the CellHasRadioButton property to set the cell of radio type. To change the state for a cell you can use the CellState property. To add or remove a cell to a given radio group you have to use CellHasRadioButton property. The following sample group all cells of the first column into a radio group, and display the cell's checked on the radio group when the state of a radio group has been changed:

Private Sub ExplorerTree1_AddItem(ByVal Group As EXPLORERTREELibCtl.IGroup, ByVal Item As EXPLORERTREELibCtl.HITEM)
    With Group.Items
        .CellHasRadioButton(Item, 0) = True
        .CellRadioGroup(Item, 0) = 1234 ' The 1234 is arbirary and it represents the identifier for the radio group
    End With
End Sub

Private Sub ExplorerTree1_CellStateChanged(ByVal Group As EXPLORERTREELibCtl.IGroup, ByVal Item As EXPLORERTREELibCtl.HITEM, ByVal ColIndex As Long)
    Debug.Print "In the 1234 radio group the """ & Group.Items.CellCaption(, Group.Items.CellChecked(1234)) & """ is checked."
End Sub

Note: A cell is the intersection of an item with a column. All properties that has an Item and a ColIndex parameters are referring to a cell. The Item parameter represents the handle of an item, and the ColIndex parameter indicates an index ( a numerical value, see Column.Index property ) of a column , the column's caption ( a string value, see Column.Caption property ), or a handle to a cell ( see ItemCell property ). Here's few hints how to use properties with Item and ColIndex parameters:

Group.Items.CellBold(, Group.Items.ItemCell(Group.Items(0), 0)) = True
Group.Items.CellBold(Group.Items(0), 0) = True
Group.Items.CellBold(Group.Items(0), "ColumnName") = True