This is a very hard question because the experience of each entrepreneur will be different based on what they are seeking to gain as well as what they contribute. Develop a list for yourself of what you seek from a mastermind and then start looking. House of Genius is one I have worked with that is hugely collaborative, though not officially labeled a mastermind group. Meetup has lot of groups with specific targets but again, it's best if you know what you seek to gain, also how you wish to contribute, before searching for a mastermind.