I'm a member of a very good mastermind and it is invaluable. There are 100 people in it, which makes it an incredible resource, but hard to keep up at times. Small ones might be hard to find the right people, and if this gets traction, a lot of people would want into it, meaning lots of small master mind groups.
Tricky to organize, but with the right people in it, I would be interested as well.
I have lots of ideas on how to make this into a startup and to organize this. Even have the domain mentormatcher.com for it. No time right now though.