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.
Source: Ultra Prism FAQ (2018-01-19), TPCi Rules Team (2018-02-01)

Relevant cards

Note: All applicable cards may not be listed below.
SM5_70