Not true, I've checked it with debugging repeatedly. In fact what happens is:
Example mob worth 1000 xp.
One person killing one mob gets 1000xp.
Six people killing six mobs get 6600xp (which equates to 1100xp per player before normalization).
If all six players are the same level and are class/race combinations without any XP modifier then all six players will get 1100xp each.
However, if the group is again the same level, but has some penalties etc, it works out like this:
Code:
Race Class Per Mob For Six
--------- ------- ------- -------
Human Cleric 162 974
Ogre Warrior 174 1042
Halfling Rogue 142 851
Erudite Wizard 180 1083
Barbarian Shaman 171 1026
Half-elf Ranger 271 1624
--------- ------- ------- -------
Total 1100 6600
So except for the Human Cleric and the Halfing Rogue, all members of the group earn more XP for killing 6 mobs while grouped than they did killing one mob solo.
The benefits of solo'ing vs. grouping depend on a lot of factors, but from a raw XP perspective, if the group can consistently kill 6 mobs in the amount of time the solo'er kills one mob, the group will earn faster XP over time.