What I have done (at least at non-FS properties) when I'm the organizer is to ask others who are traveling if any of them have MR #s. I haven't been going through Marriott's Group desk as they tend to add cancellation restrictions, but won't reduce the price at all. (So they don't really add any benefit except maybe some more points for me.) Since I can only receive points for upto 3 rooms, I ask the front desk person to add one of the other person's MR# to 3 other rooms, I do this until I either run out of rooms or people with MR#. They normally all will get their points.

So what you might do is ask your organizer if s/he would be willing to have your MR# associated with three of the rooms. Note that I still pay for all my rooms.