Specifically a weekly conference group.
Here's a system a masterminded with Joel of Buffer (pun intended).
General rules: Break the session into 3 parts and set time limits. Stick to one mastermind group partner for each session. Take it in turns for each part of the session.
Prep: Have your achievements and challenges list ready before the meeting
1. Spend 10mins celebrating "achievements" from the previous week. Can be big achievements or small. doesn't matter as long as they're meaningful to you.
2. Spend 5 mins doing "reflections". These are your reflections on the challenges you had last week.
3. Challenges: 25mins. Express your "challenge" and let the other person lead you to a resolution through a series of guided questions as part of a discovery.
Note: It is imperative that your partner never gives you an answer but only asks guiding questions.
Joel's company uses this system amongst all his staff and the company just got valued at $60m. You can read about it on techcrunch or on his blog.
If you want more detail on how to run a successful MMG or want to hear the story of how he and I spent a year doing this every week for a whole year. You know where to find me. :)
Over the years I've started a few mastermind groups (teleclass, Skype, or in person), have helped others start them, and have "taught" webinars about them, too.
Weekly groups, that happen at the same day/time seem to be the most helpful. Helps that each week another person is "the leader" -- not really a leader, but keeps the group going forward.
What I can offer you (for free) are articles 2, 3, 4 and 5 on this page.