Brute force method:
Function grpNumbers(ByVal valIn As String) As String
Dim sTmp
Dim i As Integer
Dim j As Integer
j = 1
For i = Len(valIn) To 1 Step -1
sTmp = Mid(valIn, i, 1) & sTmp
If j Mod 5 = 0 Then
sTmp = " " & sTmp
End If
j = j + 1
Next
grpNumbers = LTrim$(sTmp)
End Function