I believe that any character who kills someone gets the experience points exclusively, so if you think about it, the fewer characters you have, the faster they will go up in level (in theory).
I am not sure, however, if the bonuses you get for completing objectives are divided equally or given in lump sums.
No, when you kill one guy it gets spread out 6 ways whether you have 1 guy or 6. So, for example you kill a raider and get 11 points with just your main character you'll also get 11 points per person if you have 6 people along with you.
Confirming logicmonkey. take all the people along with you, you get the same xp no matter how many people you have in your party. the only xp that is not distributed to everyone is first aid, doctor & repair experience.
