If instead of using an ActiveX button, which, as Paul says, becomes the selection when you click it, you were to use a ribbon button to call the userform, you wouldn't have this problem, as the selection would remain in the table cell.
With an ActiveX button you would have to add the means of indicating to the macro which cell to process. This could, for example take the form of a pair of combo-boxes to indicate the row and column respectively. This works as long as there are no split or merged cells.
You can then refer to the selection in these boxes and create a range to the corresponding cell. e.g. as follows
Code:
Option Explicit
Private Sub UserForm_Initialize()
Dim oTable As Table
Dim i As Integer
Set oTable = ActiveDocument.Tables(1)
With Me.ComboRow
For i = 1 To oTable.Rows.Count
.AddItem i
Next i
.ListIndex = 0
End With
With Me.ComboColumn
For i = 1 To oTable.Columns.Count
.AddItem i
Next i
.ListIndex = 0
End With
End Sub
Private Sub btnUpdateP_Click()
Dim oTable As Table
Dim oCell As Range
Dim PeopleChoice As String
Dim iRow As Integer, iCol As Integer
Set oTable = ActiveDocument.Tables(1)
iRow = Me.ComboRow.ListIndex + 1
iCol = Me.ComboColumn.ListIndex + 1
Select Case True
Case Is = Me.optA0.Value: PeopleChoice = "A0"
Case Is = Me.optA3.Value: PeopleChoice = "A3"
Case Is = Me.optA5.Value: PeopleChoice = "A5"
Case Else: PeopleChoice = "C5"
End Select
Me.Hide
Set oCell = oTable.Rows(iRow).Cells(iCol).Range
oCell.End = oCell.End - 1
oCell.Text = PeopleChoice
Unload Me
End Sub