Source Result
# Owner Make Owner Car
1
 Jeremy Ford Jeremy Ford
2
 Jill Toyota Jill Toyota
3
 Jack Toyota Jack Toyota
4
 Arnold Ford Arnold Ford,Holden,Mazda,Mercedes
5
 Arnold Holden Sage Toyota
6
 Arnold Mazda Patrick Toyota,Tesla
7
 Arnold Mercedes Alex Unknown
8
 Sage Toyota Rain Tesla
9
 Patrick Toyota
10
 Patrick Tesla
11
 Alex Unknown
12
 Rain Tesla


Output must retain hierarchy that input has i.e. Jeremy must remain on top, Jill must remain the next, etc.


Code:
// Table1
let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    Grp = Table.Group(Source, {"Owner"}, {{"All", each _, type table [#"#"=number, Owner=text, Make=text]}}),
    List = Table.AddColumn(Grp, "Car", each [All][Make]),
    Ext = Table.TransformColumns(List, {"Car", each Text.Combine(List.Transform(_, Text.From), ","), type text})
in
    Ext