"When a PC kills a mob, he gets to eat the pie. When players group together and kill a mob, they have to split the pie. However, through the magic of the group bonus, for each person in the group beyond the first, the pie gets a little bit bigger (2% bigger in fact) before we pull out the knife to divide the slices."
https://wiki.project1999.com/Mechani...for_Experience
Soo.. maybe try a trio for a definite boost to exp, if there's enough mobs around.