Any character that is in a position to help the player’s character who make the roll.
The acting player must accept the help. The helping player must physically give the acting player one of his dice. The helping player must describe how her mouse is helping the other character. The character must have the appropiate Skill or Wise to help. Abilities always help abilities.
Your character is trying to lift a fallen tree to free a wounded fellow patrol guard. Other two characters try to help you lift the tree, placing each one at one end of the trunk with you in the middle. That’s a Health test, so they give you two extra dice to roll.
As a side note: In addition to purchasing the book when it came out, I purchased PDF copy. I’ve found that it is incredibly useful to have a searchable copy of the book. I’m not sure if Luke is selling the PDF directly through the Burning store, but if not, I believe it’s available on DriveThruRPG (rpg.drivethrustuff.com).