Recruitment
Social groups acquire and renew their members via recruitment. Compare proselytism.
In the initial stages of expansion, the groups usually do not accept every applicant. One of the ways to build a reasonably closed group is to accept new members after one or more existing members propose and recommend them. Such group expands along the lines of other existing social networks. Other approach is to use existing members to evaluate the applicant, like in Microsoft interview. Member evaluation can also be delegated to some team that is not part of the group itself (like in High IQ societies). Some groups may choose to easily accept a lot of people but only leave the most efficient new members after probation (discarding others).
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