SELECT group_id, name, register_date
FROM (
    SELECT  @group_number := IF(@group_id = group_id, @group_number + 1, 1) AS group_number,
    @group_id := group_id as group_id,
    name, register_date
    FROM
    (SELECT @group_number := 1) a,
    (SELECT name, @group_id := group_id as group_id, register_date FROM users ORDER BY register_date DESC) b
  ) c
WHERE group_number <= 2
Order by group_id