Art 130 (9)(a) a flight shall be deemed to be a private flight if the only
valuable consideration given or promised in respect of the flight or the purpose of the flight other than:
(i) [a hire charge]; or
(ii)[payment into a group fund of a jointly owned aircraft];
is the payment of the whole or part of the direct costs otherwise payable by the pilot in command by or on behalf of the employer of the pilot in command, or by or on behalf of a body corporate of which the pilot in command is a director, provided that neither the pilot in command nor any other person who is carried is legally obliged, whether under a contract or otherwise, to be carried.
FFF
If the costs are reimbursed by an employer as above, it doesn't matter how many people are carried (providing none are obliged to be carried). The cost sharing exemption is a separate one (para 8), and involves a reimbursement of costs by the passengers.
Note that from the point of view of aviation law, it doesn't matter if it's a business trip or not. However, if the employer pays for non-business flights, the Revenue is clearly going to take a keen interest.