I've got Vollinger in my party and think he's fairly useless, and if he takes any of the party's XP, I don't want him anymore. The way I understand it, each character gets XP for every point of damage done, and a small bonus when a creature is killed. Is this how it works?
Vollinger is far from useless. Give him a Sharpshooter's Pistol or another fast gun, and he does fine in combat. Give him a Mechanized gun or a Warbringer, and he becomes deadly. Plus he can make revolvers and repeater rifles (which you can sell if you have no other use for them). He is only really useless if you are playing a magickal character.
As for the Divination of XP, it's a complicated process involving you getting all the XP. Your party members will level up when ever you do.
But Castaway was enquiring about the arcane and complex methods of XP allocation. Xz was divining an answer for him. Makes sense to me. Xz, deposit the payment in my Swiss bank account. Castaway, all the XP goes to the PC. The NPCs cannot gain XP by themselves. However if the NPCs inflict damage on a foe, the XP for the damage is lost. You only get the XP from the foe's death. The only way to get all the XP is to do all the damage by yourself (i.e. no followers).
If you really want followers, and want to get full XP, then make sure you're faster than they are and skilled in melee. Also, fighting in turn based combat makes killing enemies yourself that much easier. I don't recommend it all the time, but to level yourself up it works wonders.