Critical Mass / Numbers or Skills for Startup

in CW 3 Software - Coordinators Helping Coordinators, Timebanking - Coordinators Helping Coordinators

We are in the process of launching a Time Bank for Indianapolis.

We have a Core Team of about 6-8 people, working to setup a Board of Directors, and defined Coordinator roles.

We are wondering if others have experience/suggestions on what is a "critical mass" of participants or skills in the TimeBank that make a successful startup.

We are looking at Launch Projects or Community Projects, and Pitch-ins/Potlucks as a way of drawing people in. 

But, do you have any thoughts on that "critical mass" where the Time Bank begins to operate effectively because there is enough variety, service offerings to make it "attractive" or worthwhile for people to invest in?