If two or more characters are attempting to work in concert to accomplish some task or overcome an obstacle that lends itself to cooperation a character can lend a boon to another. The GM decides which character will be rolling the dice, this should be the character taking the primary action, if that is unclear it is the character or player with the most vested interest.
That character can use all of his boons as normal, depending on what the helper is doing one or more of the helpers boons can be used in th role. Which boons can be used will depend on what the helper is doing to help.
In either case still only three boons can be added to a roll (1 power, 1 percision and 1 possibility). The sidedness of each pool (d8/d6/d4) is controlled by the character actually making the roll.
For example two PCs are interrogating a suspect. One has +2d persuasion and +2d detect lie as well as his +1d cop. If this cop, which we’ll call good cop, was to interrogate the suspect alone he’d put cop into power, persuasion into precision and detect lie in to possibility. He’d probably do fine, but he has a friend, called bad cop, who among his boons has +2d intimidation. Bad cop is going to glower menacingly and occasionally snarl during the interrogation. Because of his help good cop gets to use bad cop’s +2d intimidation as his power boon instead of using the +1d cop.
You and what army?
Sometimes a PC gets an army, or at the least a squad (or army corp of engineers). Such reinforcements are considered a Prop. As such they can be used to assist the PC in things army’s are good for.
Generally 1-5 guys with basic training constitute a +1d prop. More guys or guys with more intensive training constitute a +2d prop. When you get more than 25 or so guys you are probably getting into +3d range.
When two PCs meet sometimes they will ask have I heard of this guy. In addition to the character’s own relevant boons (streetwise, etc) a character can use the boons of the character he is trying to recognize if they would seem to add to his fame. For example boons like Mob Boss, Famous or Fixer could be used by anyone trying to recognize the character by sight or reputation.
This works largely the same as the Helping Out described above.