For a running organization, this is decided by the current mission circle which includes the two links between general circle and mission circle. A mission circle will most likely define qualifications they would like to see of future members, including stakeholder representation and other expertise.
For a forming organization, if there is a general circle and no mission circle yet, one idea is for the general circle to select their leader and a delegate and possibly another member to form a mission circle of three people that can then add as many mission circle members with qualifications of their choice.
For more information, check out our handbook Many Voices One Song section 2.5.3