A careful reading of this dev post by Rich Waters in 2002 suggests that pets should steal xp even in groups if they do 50%+ damage to the mobs:
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				How is this different than the old way for non-Dire Charmed pets?  
 
In the old scheme, if a pet did more than half of the damage to a monster, it took half the experience reward. In the new scheme, pets take zero experience unless no player does damage. If no player does any damage, then the pet takes 75% of the experience reward. 
 
This means that it's much easier to make sure a pet doesn't take any experience from you or your group. In most situations where a pet would have taken half the experience before, it now takes no experience at all.
			
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 http://everquest.allakhazam.com/story.html?story=960
Now I'm not 100% sure about this and there might be posts that I'm not aware of that are a bit clearer. Feel free to chime in if you know any, I will keep looking as I'm able.