You were pretty close to the solution. ;) Try this:
Private Sub CommandButton1_Click()
Dim rA As Long, rB As Long
Dim lastRow As Long
rA = Cells(Rows.Count, 1).End(xlUp).Row ' returns last row of first column
rB = Cells(Rows.Count, 2).End(xlUp).Row ' returns last row of second column
lastRow = IIf(rA > rB, rA + 1, rB + 1) ' returns maximum of rA and rA plus one
If optMemberName.Value = True Then
Cells(lastRow, 1) = txtMemberName.Text
ElseIf optMemberID.Value = True Then
Cells(lastRow, 2) = txtMemberID.Text
End If
End Sub