It just isn't true that the rewards go down for being in a party. Yes, you'll spawn more monsters, but it's much easier to defeat them with a party. Just one advantage: if you die, someone is hopefully there to save you the XP penalty for respawning. There are many more.
The XP per kill is less (because the team splits the reward), but that's appropriate since you are working together. The XP amount is shared only by those who are in the area, and there's a party bonus that increases the total XP awarded. In other words, the cumulative XP is better.
The most successful players, in my opinion, are those who work in tag teams (Nameless and Nuli, for example). You can complete the module solo, but it's slower going and harder in my opinion. So, just enjoy the team play and share the loot. It's worth it.