Concatenate one column per ID

let
  Source = Table.FromRows(
    Json.Document(
      Binary.Decompress(
        Binary.FromText(
          "i45WMjQyNlHSUQpOzStRitWB852LUhNLUlOQhcITM0sy89JxCVlamOMQgpttZmpiBOT7p6WlFiELIOuBCkH0xAIA", 
          BinaryEncoding.Base64
        ), 
        Compression.Deflate
      )
    ), 
    let
      _t = ((type nullable text) meta [Serialized.Text = true])
    in
      type table [ID = _t, Stage = _t]
  ), 
  #"Replaced Value" = Table.ReplaceValue(
    Source, 
    each [Stage], 
    (k) =>
      let
        ld = List.Distinct(
          Table.SelectRows(
            Source, 
            each [ID] = k[ID] and List.Contains({"Sent", "Waiting", "Offer"}, [Stage])
          )[Stage]
        )
      in
        if List.Contains(ld, k[Stage]) then Text.Combine(ld, ", ") else null, 
    Replacer.ReplaceValue, 
    {"Stage"}
  )
in
  #"Replaced Value"

How to use this code: Create a new Blank Query. Click on "Advanced Editor". Replace the code in the window with the code provided here. Click "Done". Once you examined the code, replace the Source step with your own source.

Hi deannapi, another solution:

 

Before

 

After

 

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQyNlHSUQpOzStRitWB852LUhNLUlOQhcITM0sy89JxCVlamOMQgpttZmpiBOT7p6WlFiELIOuBCkH0xAIA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, Stage = _t]),
    Stages = List.Buffer({"Sent", "Waiting", "Offer"}),
    GroupedRows = Table.Group(Source, {"ID"}, {{"All", each 
        [ a = List.Intersect({Stages, [Stage]}, Comparer.OrdinalIgnoreCase),
          b = Text.Combine(a, ", "),
          c = Table.ReplaceValue(_, each [Stage], each if List.Contains(a, [Stage]) then b else null, Replacer.ReplaceValue, {"Stage"})
        ][c], type table}}),
    CombinedAll = Table.Combine(GroupedRows[All])
in
    CombinedAll