The sergeant player in my game warned me to not let him take Peer(Imperial Guard), especially multiple levels of it because it will make his social/leader character completely overpowered.

Peer(Imperial Guard)(X) says it will give a 10 * X bonus to Fellowship tests with the chosen group. A surface reading of the rules make it seem that then any Command test (Fellowship based) will start to get this bonus. Quickly, using Terrify, the squad becomes immune to Fear, orders always pass, Battlefield Maneuvers tests always pass, etc.

Digging down, I'd start to think that it wouldn't really affect Command checks, but more like Charm/Deceive/Inquiry type tests with Guard, and then I'd even start to reduce the benefit the farther the character gets from interacting with his own squad/platoon/etc.

Any thoughts or experience with this so far?

The Peer (X) talent contains a caveat that there must be a real, concrete reason why they would be able to take the talent. What the Peer (X) talent represents is that faction/group has taken a liking to the character, or the character is a well-respected member of that group. As a GM, I would require that the character have done something so great that everyone in the Imperial Guard has a newfound sense of respect for the character - not just something to be taken because your Sergeant feels like it.

It is worth noting, too, that the talent only gives you a flat +10 bonus to interaction tests with the group. So to get a crazy amount of bonuses, you would have to take it several times. This is rules as written, because doing so should represent the character increasing his reputation with that group - again, by doing things that would earn him the respect the talent implies.

The talent's description also recommends the GM use this as an excuse to ask for favors or request assistance from the character. Being so thought highly of, surely he wouldn't mind being dropped into the tent of the local Warboss...

The squad in question has been pivotal for the success of the platoon (obviously, as they are the PCs!) so I think an initial level of Peer that works on the Platoon level seems within the scope of the rules. I would require justification for any further levels of Peer (Imperial Guard).

Would you allow Peer to affect Command tests though?

Absolutely I would, though limiting it to only your platoon is a good compromise in my book. Keep in mind, though, if your sergeant is trying to command the Lieutenant or other sergeants in the platoon, that +10 could be canceled out with negatives pretty quickly. I don't think it will be a problem. If it does become too overpowered with the Sergeant's high Fellowship and Command, then I would still let it fly. Obviously he has built his character this way, so why not let him play the way he wants?

Because if he is sinking all of his XP into Fel and Command, he isn't putting it into Common Lore (Tactica Imperialis), Willpower, Ballistic Skill, Toughness, Dodge...

Yeah, the sergeant as it stands has too many things they need to be good at (melee, shooting, social, AG for dodge, intelligence for tactics..) So if he puts too much into peer he'll really suffer in other realms.

also is peer imperial guard a thing? doesnt make much sense to me that in one move you can become loved by the ENTIRE guard. surely it would be specific to the part of the guard, e.g.

Peer: platoon 14

peer: Ratlings of regiment x

Peer: leman Russ squad x

you get me? also there are nearly always rivalries within the regiment, so being peers with one group may not allow you to be peers with another group. if B and D platoons hate each other then its gonna be hard to be peers to both!

I think "Peer (Imperial Guard)" doesn't fit in a game where you play... the Imperial Guard!

It must be something copy&pasted from Rogue Trader, Deathwatch or such.

I wouldn't allow it.

"Peer (Comissariat" or "Peer (Mechanicus)" makes more sense. "Peer (Ratlings)"... could be!

I think he is considering less of Peer (IG) and more of Peer (Platoon X) which I think works, makes sense, and isn't overpowered.

If you feel your Sergeant has min/maxed his Command to the point where it is a problem, have someone shoot at him. Or charge him. Or pin him. He will quickly come to regret sinking all his XP into that one aspect of his character.

How is getting a +10 bonus on interaction tests with a certain organization OP? How is this any more op than the other talents that pretty much does the same? Peer: Inquisition in a DH game? If the problem is taking it multiple times maybe do the same as with Psy Rating, increase the cost by a factor of the next level. So for a Sgt. thats 300, then 600, then 900 etc. That should make it less unbalanced.

Having it limited to only a single platoon makes it potentially worthless, depending on the GM and story.

The talent can be justified by having rank, or even awards that are obvious to members (think Silver Stars and medal of Honor). Or as intangible has having a certain reputation of being a hero.

Peer Adeptus Mechanicus already exists btw.

If there is an issue with Command it's the skill that's the problem rather than getting the bonus, after all it's fairly easy to become absurdly good at a skill in the system as it is. Command generally only works on soldiers who are of inferior rank anyways, so it's not like you can waltz up into the Commanders tent and start commanding him. And if someone decides to invest thousands of XP in becoming über commander being able to thwart Fear isn't necessarily breaking the game or autowinning all their combats.

Welcome to Hero of the Imperium status!

This is how the legend of a certain Ciaphas Cain started.