Let me add my experience:

~3 years working with 3DGS
~20k USD invested in programmers to help me with my system.
~2000 people-hours to create the system.

Trust me, "for free" just isn't going to happen unless you already have friends willing to work with you. The Massively in MMP not only refers to the world size, but the amount of time, money, and aspirin you'll need to make it happen.

The problem is always the same: when it's for free, there is no incentive to persevere when the going get's tough (and here, the going get's tough right away). When you are up at 3am in the morning trying to figure out why your client is going right instead of left and you have a term paper due the next day and you've just come home from a grueling 12 hour shift at work and the kids or your parents are nagging you to death and on top of that you are doing this for FREE for someone else? Can you see why so many of these community MP projects have failed? You wouldn't go and work for McDonalds for 100 hrs a week for free, how is this any different?

My best advice is to find one or two like minded individuals and try for something simple learning experiences. Get four people in there... create a chat room... create a user/pass login system. As you create these small componenets, the "how" of getting 100 ppl in the world will slowly develop as well as the know-how of managing a team as well as perhaps he compensation to keep people around longer than a day.