I don't have a source but I'm pretty sure vanilla has an exp bonus for groups of 5. If you are grinding single targets your leveling time will be split up into 3 categories:
1) Kill time will be ~1/5
2) Recovery time will be ~1/5 (because kill time will be faster you'll spend less mana/hp)
3) Transit time between mobs will be roughly the same
So with single Target grinding I think it would really tough to achieve comparable rates to solo or duo grinding. You might be able to make something work if you could aoe grind efficiently? Or if you use some instant cast grinding method like warlock CoE, CoA and your group never stops running and continues to pop instant dots. I believe kargoz mentioned this technique on his stream the other day for duo leveling. Could work in 5 box combo maybe?

Duo grinding I do on average 4 kills/min. To hit similar rates you would need 8-10 mobs/min with 5ppl. So that would be very tough if you were killing things 1 at a time