Perhaps I’m not understanding what you want to do, but you should already be able to do that. You can have as many “positions” in the directory as you want, and each position can have any number of members. Also, a member can belong to more than position.
So, for your situation, I would set up one “position” for the committee chairs, and one position for the committee members. You could do that for every committee. By doing it that way, you also make it easy for somebody to contact only the committee chairs (since they have their own position) without it sending a message to all committee members. Also, the chairs can be included in the committee members position as well, so if you do want to send a message to all members, the chairs would get that message as well.