Use the AutoWidth property to arrange the columns to fit the entire group's content. The AutoWidth property doesn't change the column's width. Use Width property to change the column's width at runtime.

Private Sub autoSize(ByVal t As EXPLORERTREELibCtl.Group)
    t.BeginUpdate
        Dim c As Column
        For Each c In t.Columns
            c.Width = c.AutoWidth
        Next
    t.EndUpdate
    t.Refresh
End Sub