<!-- just use textAlign="center" -->
<Table>
  <Thead>
    <Tr>
      <Th />
      <Th>First name</Th>
      <Th>Last name</Th>
      <Th textAlign="center">Participation</Th>
	</Tr>
	</Thead>
	<Tbody>
  	{participationData.map((participation, i) => (
      <Tr>
        <Td>{i}</Td>
        <Td>{participation.firstName}</Td>
        <Td>{participation.lastName}</Td>
        <Td textAlign="center">{participation.participation}</Td>
	  </Tr>
    ))}
  </Tbody>
</Table>