Battlerager Armor SCAG p121
When you choose this path at 3rd level, you gain the ability to use spiked armor as a weapon.
While you are wearing spiked armor and are raging, you can use a bonus action to make one melee weapon attack with your armor spikes at a target within 5 feet of you. If the attack hits, the spikes deal 1d4 piercing damage. You use your Strength modifier for the attack and damage rolls.
Additionally, when you use the Attack action to grapple a creature, the target takes 3 piercing damage if your grapple check succeeds.
For enemies who have resistance/immunity to "bludgeoning, piercing, and slashing from nonmagical attacks", would they still be damaged by the battlerager if the battlerager grappled them instead of attacking with a weapon? Here's what it says what the Grapple action is in the phb.
Grapple PHB p195
When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you're able to make multiple attacks with the Attack action, this attack replaces one of them. The target of your grapple must be no more than one size larger than you, and it must be within your reach.
Using at least one free hand, you try to seize the target by making a grapple check, a Strength (Athletics) check contested by the target's Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). You succeed automatically if the target is incapacitated. If you succeed, you subject the target to the grappled condition (see the appendix). The condition specifies the things that end it, and you can release the target whenever you like (no action required).
The way I see it, while a grapple is a "Special Melee Attack", it is not an attack because it lacks an attack roll, instead being a check instead. Thus if a werewolf or similar creature with resistance/immunity to piercing was grappled by a Battlerager, they'd still take the full (If measly) 3 piercing damage, even if the Spiked Armor isn't silvered or magical.
