Using Group-Ungroup in a protected Excel sheet

Jyters 

If you want to do this for all worksheets in the workbook:

Private Sub Workbook_Open()
    Dim ws As Worksheet
    For Each ws In Me.Worksheets
        With ws
           .Protect Password:="Secret", UserInterfaceOnly:=True
           .EnableOutlining = True
        End With
    Next ws
End Sub

If you want to apply it to several but not all worksheets, use something like this:

Private Sub Workbook_Open()
    Dim ws As Worksheet
    ' Modify the list of sheet names as needed
    For Each ws In Me.Worksheets(Array("Sheet2", "Sheet4", "Sheet6"))
        With ws
           .Protect Password:="Secret", UserInterfaceOnly:=True
           .EnableOutlining = True
        End With
    Next ws
End Sub

Mick_Kitcher 

I have attached a very simple demo workbook. Does it work for you if you allow macros when you open it?

I think I found a solution in a very old webpage:

https://www.mrexcel.com/board/threads/allow-grouping-option-on-a-protected-worksheet-workbook.479873/

Modified VBA Code as follow, and :

Sub Workbook_Open()
    With worksheets("yoursheetname")
        .Unprotect "PASSWORD"
        .EnableOutlining = True
        .Protect "PASSWORD", contents:=True, userInterfaceOnly:=True
    End With
End Sub

(I've bolded the "yoursheetname" and capitalized + bolded the two "password" so that it's easier to notice them) 

I've also changed the original "private sub" to "sub", because it needs to be activated everytime I re-open the workbook, and thus I made a button that is linked to this module, and it does need to be clicked once after every re-opening of the workbook.

For some reason it does not work in the "private sub" format without the button-clicking, no idea why (still new to VBA), but anyhow, It is working fine for me for now (currently using Excel 2019).