Personally I don't see why I should let anyone get cheap flying at my or a groups expense.
The group is not losing out because the group member is still paying for the flights as if
he was flying.
I am not saying this is honest but I don't think it is (necessarily) illegal.
Look at another angle: how many group planes go off somewhere and the pilot (a group member) gets a contribution from passenger(s) under the PPL Cost Sharing Scheme? I don't think anybody complains about that.
And even if it was illegal (due to insurance issues) it would be unenforceable because any logbook entries will be made only after the flight has landed.