Usually for charity, various challenges have been completed regarding how many people you can fit inside some vehicle or other. An impressive choice is a Ford Transit van, though the more comical options have included Minis, Vauxhall Novas, and Smart cars. As a joke, this is hilarious. As a means of actually moving people about, it is ludicrous. Use London coach hire and ride in comfort and style.
If anything is worth doing, it is worth doing it with your friends. Being in good company when you are enjoying yourself always amplifies the experience. However, getting there and back will always be a hassle. There is something to be said in favour of overcoming such problems with ingenuity, but there is also something to be said for personal space and avoiding crippling cramps.
Your options are somewhat limited if you are trying to get anything more than five people to a venue and back. You can split the group, but that ruins the dynamic and depends on at least two people knowing where they are going. Also, the difficulty and cost of parking multiplies. You could squeeze everyone into one vehicle, but this is incredibly uncomfortable after the first 10 minutes and is also very dangerous. After that, your options are to not go or to get a coach.