We got this question one day from an event planner, and then the next day from their client. The question was does the planning agency or their client need consent. The answer was that as long as the end-user (the initial business that wanted things sent) has consent of some kind, the planners should be able to send of behalf of their client. However, ensuring that express consent is obtained is the safest idea, since that is the easiest to prove. So re-confirm campaigns and express consent requirements on the forms are the safest way to advance for business relationships like this.
Remember that both the sender and the person on whose behalf the message is being sent should be identified clearly under CASL regulations.