A 3.5 tonne Sprinter is able to be a total weight of 3.5 tonnes (3,500 kgs) on the road. This includes the driver, passengers and fuel. ie. the total weight of the vehicle if it was stopped at the side of the road and weighed.
This vehicle can be driven with a B licence.
The empty weight of a van like this is about 2.3 tonnes. So you have about 1.2 tonnes to play with.
1.2 tonnes = 1,200 kgs.
Allow 80 kgs per person.
100kgs for a tank of fuel.