An ideal number of people to start a mastermind group is 10-12. Invariably, a few flake out and don't commit the time and usually its because they don't want to get out of their comfort zone. Also, over time, people move or leave town.
Consistency is key when selecting a date and time of the week to hold your group meetings. People tend to follow a routine, so having it at the same time and place helps with regular attendance.
Library meeting rooms are a great place to hold a meeting. Any place you choose, be sure that it is a quiet place with very few distractions. Houses are ok, but turn the phone off and keep the kids locked up.
The initial mastermind group meeting should give an overview of what is to be expected for the meetings. It should also establish the ground rules of how the process works. People should be encouraged to show up every time unless of an emergency or something unavoidable.
Think and Grow Rich written by Napoleon Hill is a fantastic book to use for the 1st several weeks of the group getting together. The book instills a great basic skill set and gets people use to using their higher mental faculties instead of just their senses. It also gets peoples desire fired up.
The optimal length of time for a mastermind group is 60 to 90 minutes. Diverse opinions add the overall value of the group, do not let a single individual do most of the talking.
Each person should come to the meeting with the idea of contributing to others. Being about others in the mastermind group is powerful because it creates a situation where each person has a group contributing to them and their life.