How to convert part of array column to text, separated by commas?

Change references where required.

Sub Maybe()
Dim arr1, i As Long, a As String
arr1 = Range("A1:B" & Cells(Rows.Count, 1).End(xlUp).Row).Value
    For i = LBound(arr1) To UBound(arr1)
        If a = "" Then
            a = arr1(i, 2)
                Else
            a = a & ", " & arr1(i, 2)
        End If
    Next i
Cells(1, 4).Value = a
End Sub