Interesting fact found on the PWI Forums:
The total exp for the party per kill goes up as the number of members increases, the experience per kill per player goes down, but as long as everyone in the party kills at the same (or similar) rates the average experience over time in a party is way higher than solo (for a full party on the order of 50-60% more experience). There is no way to earn less exp in a party than solo unless someone isn't pulling their weight in the team.
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In summary, even though you get less xp per kill, the total xp for the party increases, so assuming you double the rate of killing with a partner, your rate of xp gain will go up. This explains why Zhen parties are so popular and successful, since the total party xp is increased AND the rate of killing is massively increased.