Add users to new question email

When a user asks a question on my forum, I wanted to be able to add user email addresses to be notified by email of the new question.  I have multiple categories and for each category there is at least one forum leader.  I would like each of them to get a notification email when a new question is posted to their category.  Which hook would allow me to add user emails to the new question notification email that goes to the admin?