"If they make a profit from giving lifts to passengers, they could be classed as using their vehicle for 'hire or reward' which would invalidate their cover."
All private car insurance policies prohibit "passengers carried in the course of a business carrying passengers" or for "hire or reward".
The idea of using your car for hire or reward is particularly important if you are considering using Uber, a smartphone taxi company who connect passengers directly with drivers through a mobile phone app, rather than using a centralised booking service like a traditional private hire cab service.
Answer: If you are driving the minibus in the course of your employment, this essentially would mean you would be driving the vehicle 'for hire or reward' and, as such, different rules apply.
If you have held a full driving licence since before January 1, 1997 (which would be shown as Group A or Group B for automatics on the old style green/ pink licence, or as category B and D1 on a pink and green or photocard licence) to drive a minibus which has nine or more seats 'for hire or reward' you would need to obtain a passenger carrying vehicle category (D1 or D) on your licence by taking an additional driving test.