Copy the following into a Module,

Code:
Public Function Conc(Fieldx, Identity, Value, Source) As Variant
  Dim cnn As ADODB.Connection
  Dim rs As ADODB.Recordset
  Dim SQL As String
  Dim vFld As Variant
  
  Set cnn = CurrentProject.Connection
  Set rs = New ADODB.Recordset
  vFld = Null
  
  SQL = "SELECT [" & Fieldx & "] as Fld" & _
        " FROM [" & Source & "]" & _
        " WHERE [" & Identity & "]='" & Value & "'"
  
  ' open recordset.
  rs.Open SQL, cnn, adOpenForwardOnly, adLockReadOnly
  
  ' concatenate the field.
  Do While Not rs.EOF
    If Not IsNull(rs!Fld) Then
      vFld = vFld & ", " & rs!Fld
    End If
    rs.MoveNext
  Loop
  ' remove leading comma and space.
  vFld = Mid(vFld, 3)
  
  Set cnn = Nothing
  Set rs = Nothing
  
  ' return concatenated string.
  Conc = vFld
End Function
Then call the fucntion in a query.-

Code:
SELECT Conc("ConcatField","Byfield",[Byfield],"Table/Query") AS Field
FROM Table/Query;
Thanks

AOG