Do we keep people? There will always be an attrition rate – people move cities, they get sick, their parents get sick, they get more responsibilities at work or whatever.
These are all “external” reasons. There are also “internal” reasons (the two can of course amplify each other’s impacts) around failure of the group to efficiently achieve its goals, unmanaged conflict with other people in the group (see tyranny of stucturelessness) etc etc.
see also Legitimate Peripheral Participation
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