x2Power Huddle
As long as this Pokémon is on your Bench, your Passimian's attacks do 30 more damage to your opponent's Active Evolution Pokémon (before applying Weakness and Resistance).
Rock HurlThis attack's damage isn't affected by Resistance.
If I have multiple Passimians in play, does each of their "Power Huddle" Abilities add 30 more damage to my Passimian's attacks?
Yes, Passimian's Power Huddle ability is stackable. Each Power Huddle does 30 more damage in addition to any damage Passimian's attack already does.
If the Defending Pokemon is currently protected from effects of attacks, do I apply Resistance to Donphan's "Rock Hurl" attack or not?
No, because the phrase "Don't apply Resistance" is part of the damage calculation, not an effect of the attack.