I had not heard of a mastermind group until you mentioned it hear. Intrigued, I looked up the definition and found it to be interesting.
Judging from my own experience, it would seem that awareness is lacking more than anything. Second, you would want to have a tightly controlled set of users involved, making the list of potential candidates very short.
These two problems could be re-mediated with certain online tools, such as Google+, LinkedIn, and other online congregation networks where you could look for people with interests that would predispose them to joining such a group.
I would find the opportunity to join such a group interesting, so maybe I should help you out!