I've never understood how the party affects it - especially the luck factor. I'm not sure anyone really knows the answer to that one.
In Doom, being in a party has no effect on the points you get, it's all individually based.
It is the individuals luck- luck increases your chances of getting a drop.
Example-
Each mob is worth X points. When people do damage to a mob, the top 20 people get points. The points are divided out to the top damagers. The more damage you do, the more points you get.
Your luck increases your chance to get a drop.
Example-
You have 3000 points which equates to a 30% chance for a drop (I am completely making up these numbers) and you have 2000 luck. 2000 luck is a 68% chance to get a bump, so, lets say it increases your chance to get a drop by 68%, making your chance to get a drop 50.4% (30% * 1.68).
Just to repeat, I completely made up those numbers to illustrate an example to show in general it works.