In reply to spidermonkey09:
I organise a trip to Europe every year for my mountaineering club and we take anywhere from 11 to 30 people, and I use a mixture of the university transport available & hire new stuff in as well.
31 people - Uni 9 Seater, Hired Van, 5 Cars - left Nottingham at night and drove all night arriving in the morning.
25 people - Uni 9 Seater, Hired 9 Seater, Hired Van, Car
11 people - Hired 9 Seater, Uni Van, Car
The vehicles are all insured under the university policy (your uni will have one if it lends out vehicles to students) at a small additional cost. The biggest cost (behind fuel) is hiring externally, hence why I use the uni vehicles when possible.
I looked into getting a few D1 drivers and hiring a 17 seater, but it worked out more expensive.
Any more questions about more specific organisation of these sorts of things, then just PM me.