yeah as a volunteer you can, if you hire it you'd have to have a d1 category...
Confusingly, the DLVA's definition of "hire or reward" doesn't seem to apply to the vehicle but the driver.
As far as I can make out:
If the driver is paid to drive the minibus, they need the D1 category to do so.
Even if the group pay the driver just the out of pocket expenses of hiring the minibus and fuel, the driver needs the D1 category.
If the driver rents the minibus and pays for fuel, all out of their own pocket, they
should be able to drive on just a car licence (though it's always worth checking with the DLVA).
Even if the DVLA say it's ok to drive on a car licence in the circumstances laid out in the query, the hire company can still insist on D1...
The rental agreement would likely prevent someone paying for the hire of the vehicle and someone else driving it.