When multiple PCs take action together, the party chooses one PC to lead the action. Each other player then describes how their character collaborates on the task. The leader makes an action roll as usual, while the other players make reaction rolls using whichever traits they and the GM decide fit best.
The lead character gains a +1 bonus to their lead action roll for each of these reaction rolls that succeeded and a -1 penalty for each these reaction rolls that failed.