What about integrating group organizer tools to keep organizers hanging around and using you. Like itenerary tools, ability to send out emails to the group about changes, tools for splitting up costs evenly, that sort of thing. If they use you to organize, they might use you to book.

Also, question: you say it's difficult keeping track of commissions; if you did keep track of them well would that be enough money, or are you looking for an alternative revenue stream?