data transformation in power query

1. Group your data by the first column, and for the aggregation use the ALL ROWS aggregation and call it "AllRows"
2. Add a custom column, call it Names, and use Table.Column([AllRows], "Name") as the formula.
3. Expand the new Names column but as Values (not rows) and select the comma as your delimiter.
4. Select the ID and Names column and remove other columns.

Full M query (my data came from an Excel table which explains the source)

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Id", Int64.Type}, {"Name", type text}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Id"}, {{"AllRows", each _, type table [Id=number, Name=text]}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Names", each Table.Column([AllRows], "Name")),
    #"Extracted Values" = Table.TransformColumns(#"Added Custom", {"Names", each Text.Combine(List.Transform(_, Text.From), ","), type text}),
    #"Removed Other Columns" = Table.SelectColumns(#"Extracted Values",{"Id", "Names"})
in
    #"Removed Other Columns"

workbook is here.