You get a percentage of normal XP from kills in your party. More than XP/n (where n=# of party members), but less than 100%. It is significant - like 80 or 90% in my 2-player game - and does turn horde night into an XP bonanza. I'm alright with that, considering the amount of work that goes into getting the base ready, but I do wish the multiplier were configurable. I haven't been able to find the parameter in the XML nor serverconfig, only the shared XP range.
It would be nice if the shared XP were more nuanced than "every party member gets X% of the XP", but adding that nuance is easier typed than coded. A full-on proportional-to-damage-dealt award (maybe with a small 10% spiff to non-participating party members) would be great. You do 60% dmg to zombie X, I do 40% dmg, you get 60% of XP, I get 40% XP, and Sir NotParticipatingInThisFight over there gets a 10% XP award for hanging out with cool kids like us. But that would be hard to code and probably not worth the effort.
Maybe a little easier is a 2-tier award system. If I do 100% of the damage to the zombie, I get 100% XP and party members get their 10% spiff. If any other party member contributes, then XP is divided amongst the entire party using a formula similar to how it is now. So no need for per-bullet damage tracking, just a toggle when Player 2 enters the fight. This at least tones down the XP bonanza for the party if I'm over at my corner of the base mowing down radiated with my M60 while they're diligently watching blade traps spinning 'round and 'round. Also the one-off kills when meandering about town - if I blow its head off, you get 10% and BE HAPPY WITH THAT.
Just adding the current multiplier into the UI so we can pick from a couple of options would be great, though. Or at least expose it in XML files. Or show my stupid face where it already lives...