Play with more than 1 player in Dungeon Defenders (demo)

I was playing in a 3-man group last night: Squire, Huntress, and Monk. We set up the same defenses on all 3 entrance points to the map. Squire walls with Huntress proximity/inferno traps in front, and whatever auras the Monk felt like using (not important to this example).

When the waves came, each of us defended one section. The Squire and Huntress spent most of their time repairing defenses (especially the Huntress's traps, which replenishes the quantity), and the Monk spent all his time fighting the enemies. The end result was that the Monk had the most kills, but let all his defenses die and leaked enemies through to the crystal.

The Huntress and Squire, by comparison, had no trouble holding off all the enemies, and the Squire even held off the Ogre. The Monk was not a team player, and leaked because he didn't repair the Huntress traps. The Squire kept the Huntress traps replenished and didn't even break a sweat.

tl;dr: The key to playing Huntress effectively is to have your teammates repair your traps. I'm pretty sure that's the crux of it.

For the PC version of the game, experience is shared for all kills. So all kills will give all players the same amount of XP, regardless of who killed them.

The Experience page on the wikia supports this:

On the PC version, experience from killing creatures is shared among the entire party.