Not so, there is one very likely reason: if the aeroplane isn't maintained to public standards (and most group owned aeroplanes aren't), the legality would then depend on what Weeman pays for the privilege. If he pays more than his half of the direct costs for the flight (or pays anything towards the monthly/annual costs), the flight would constitute illegal public transport!
I am not proposing he flies or pays anything different than he does already - if he is paying for things he should not be paying for, then he (or the group member) is illegal anyway!