If a character wishes to help a character on a Skill roll, they can do so in different ways, each character can choose one of two ways to assist.
The character performing the roll is considered the Leader.
Each player can either:
Backup
Roll the same Skill, the Leader takes the highest final value of all players rolling that Skill as if performing that themselves.
Assist
Roll a different Skill, and explain how this helps, if the roll is 15 or above, the Leader may add a d4 to their roll.
For every 5 above the 15 DC, that dice size increases.
Each player Assisting must roll a different Skill to each other.