property Columns.Count as Long

Returns the number of objects in a collection.

TypeDescription
Long Counts the Column object into the collection.

You can use the following samples to enumerate the group columns:

Dim c As EXPLORERTREELibCtl.Column
With ExplorerTree1.Groups(0)
    For Each c In .Columns
        Debug.Print c.Caption
    Next
End With

Dim i As Long
With ExplorerTree1.Groups(0).Columns
    For i = 0 To .Count - 1
        Debug.Print .Item(i).Caption
    Next
End With