I looked around a bit but I couldn't find a post about this.

As written:

Nemesis Hammer Hand: The Psyker hardens his muscles and brings down the wrath of the God-Emperor of Mankind in powerful strikes with his melee weapon. This is a Psychic Power that requires a Challenging (+0) Willpower Test., requires a Half Action to manifest and is a Free Action to Sustain. While active, this power adds 5 to the Unnatural Strength trait of any Grey Knight allies within 10 metres.

My question: Does it stack with itself if multiple Grey Knights manifest the power? Eg. If five Grey Knights use Nemesis Hammer Hand, do they all get a +20 Unnatural Strength boost? My gut tells me no, but then it could be that thing that truly makes the Grey Knights into feared opponents by heretics and daemons alike. And if memory serves, Grey Knights don't deploy en masse.

What is your interpretation of the Psychic Power?

I would have to agree with your gut on this. 4 GKs swinging with +20 Unnatural Strength is likely enough to require Grandfather Nurgle himself to take a Fear test.

On the other hand, in the table top game a unit can cast it on itself and then any independent characters that have joined the unit can cast it as well, stacking the strength bonus. It can and does get a bit dumb and out of hand sometimes.

i would also say no, thou if that one inquisiter who has that power was with them i would indead bump that up to +10...just seems to fit how the tabletop game works a bit better since the whole squad is considered one psycker( i think this is mentioned in some of the books as well)

and to how broken that hammerhand combo can get? i once had a squad of strength 7 knights eat a baneblade in one round...before my hammers got in to boot. my opponite was less than happy.

Grey Knights should have rules for combining psychic strength to make powers better. As far as multiple people using the power, look in the core book psychic rules. There should be something about stacking or combining psychic powers. As far as I remember, they don't stack. I don't have my book to site pages or anything, but it should be in the general rules.

The answer is if they get enough overbleed and if they have the right talents then yes you could have 1 or maybe 2 of the Grey Knights with +20 to their strength They would all need to have both Focus of the Justicar and Brotherhood of psykers, to make it happen, which they get both by rank 3, also at that time they do get Tactical Dreadnought Armor. aka terminator armor. On the good side of things there is no overbleed in both Deathwatch and Black Crusade so a whole group of +20 strength grey knights would be out of the question. however FFG has not to my knowledge done anything for overbleed so we are safe for now. For reference grey knight combining of powers (Daemon Hunter page 119, Brotherhood of Psykers and Focus of the Justicar page 120).

"Modifiers, Characteristic increases, and other benefits generated by psychic powers do not stack—only the highest applies." pg 209.

So no, presuming more than one knight was sustaining it, it still wouldn't increase it's benefit. However, presuming more than one knight is encountered, it would free up the rest of the squad to sustain/cast something else, without having to carry the burden/risk of of sustaining a power, or the psychic strength decrease for sustaining more than one.

Far as not deploying en masse, I have no clue. I've stayed a fair bit away from Grey Knight fluff since... Ward. However, I suspect they, like most space marines, deploy in at least squads.

As for the effects Daxium mentions, those are from the rules for Grey Knights for Dark Heresy/Deathwatch. They're meant for player characters and, presuming you're just using these for enemies, don't bother with them. The Grey Knights in BC are scary enough for what they're meant to be, which is a big scary thing to kill or flee. Including complicated rules for combining psychic strength and such is, IMO, overly convoluted for something that's just there to be big, scary, and either killed or ran away from.

I do find it odd that the caster does not benefit from the power, though.

Reverend mort said:

Grey Knights will deploy in any strength of numbers that is required, assuming the bodies are available. This goes from a single GK accompanying a Malleus inquisitor to near chapter size engagements.

I don't understand why people don't like Ward's fluff either. It's in line with all of the other 40k fluff I've read. But that is a discussion I won't really go into with this thread.