Use the FirstVisibleItem, NextVisibleItem and IsItemVisible properties to get the items that fit the client area. Use the NextVisibleItem property to get the next visible item. Use the IsVisibleItem property to check whether an item fits the group's client area. 

The following sample enumerates the items that fit the group's client area:

On Error Resume Next
Dim h As HITEM
Dim i As Long, j As Long, nCols As Long
With ExplorerTree1.Groups(0)
    nCols = .Columns.Count
    With .Items
        h = .FirstVisibleItem
        While Not (h = 0) And .IsItemVisible(h)
            Dim s As String
            s = ""
            For j = 0 To nCols - 1
                s = s + .CellCaption(h, j) + Chr(9)
            Next
            Debug.Print s
            h = .NextVisibleItem(h)
        Wend
    End With
End With