I turn on sharing for everyone.
Primary Group - tank, CC, healer, DPS, DPS, plee (powerlevelee)
Chillin - other toon other toon other toon etc
when plee is no longer gaining the bonus exp from vitality, I swap in one of the chillin toons and work on them while the previous plee starts gaining their vitality back.
If I don't need CC I swap that with an additional plee
10% per kill is cap iirc, when the plee is in the group with the primary group. When they are in the primary group I turn off sharing unless the "Chillin" group is also leveling somewhere else at their own ability level, in which case it's left on because you can't gain vitality from the group you're currently in.
If you have toons that are maxxed out it's best to use those to give vitality to others since they don't technically lose anything except potentially some aa exp they would use for glyphs.
If running two groups (even if group 2 is "killing at their ability level" it's good to take one serious DPS'r and place them in your poop group. This significantly speeds up the rate at which they kill.
In my example I took one max level max AA zerker out of the primary group, and ran them in the plee group to kill bump the poop groups overall DPS to well over 10x what the group did by itself. Leaving more than enough DPS to handle what I was fighting in the primary group since I still had tank,dps,cc (which doubles as dps), healer, plee, plee.
If you're power leveling, it's best to do it in bulk, even if you don't plan to use those classes/characters anytime soon. I personally just have one of each class and an additional berzerker.
Once a character is max level there's no reason to turn off sharing, if a character isn't in the primary group there's no reason to turn off sharing unless they too are max level/aa but even then I haven't the memory to remember to turn them off and on so I just leave them on.
Lastly, find a place you're effective at killing, and burn the world around you. Low kills per minute on yellows, versus very very fast kills per minute on dark blues - in my opinion go for the dark blues.