'Group By' operators - no PRODUCT option?

You can use whatever function you'd like with a small tweak to the code. Do the Group By using Sum and it will generate a step that looks like this:

Edit the code in the formula bar to replace List.Sum with List.Product and it will take the product over the group instead of the sum.

Sample M code you can paste into your Advanced Editor:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUXIEYiOlWJ1oIAnhGYN5xkCWExCbgHkmUDlDMM8UKgfRZwaVA6qMBQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, Group = _t, Value = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ID", Int64.Type}, {"Group", type text}, {"Value", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Group"}, {{"Value", each List.Product([Value]), type nullable number}})
in
    #"Grouped Rows"

I use this trick with other operators too. For example, Text.Combine to concatenate text rows.