they either fight the crew (the defender nominates who fights who) or they do nothing if there's no one available to fight (because they're all fighting someone else).
Reading this, I thought 'is that really right?' The idea that they simply stand around and do nothing seems a bit odd.
I had a look at Da Roolz. As often the case, they're not entirely clear and it may be that something elsewhere in the book changes things. But, according to p. 44, "A warrior who finds himself unopposed on a vehicle can attempt to damage the vehicle itself should he so wish".*
I think 'unopposed' here is open to interpretation. One interpretation is that a model is only unopposed if they are no models from the other side on the same vehicle. (This seems to be how you're reading it.) However, another interpretation is that a model is unopposed if he is not matched up with an opposing model.
On the latter interpretation, then if three people board a trukk that only has two models (driver and one passenger), it means that two of them would fight the crew and the third boarder would be unopposed, and thus able to attack the vehicle itself.
I'm not sure which of these was intended by the writers - quite possibly the first interpretation - but the second seems to make more sense than standing around and doing nothing.
*One caveat to that (further illustrating my point about the rules writing) is that this is under a heading about damaging a stationary
vehicle. However, despite that heading, nothing in the text under seems to mention the vehicle being stationary. Therefore, it's not really clear whether or how it applies to a moving vehicle.