How about groups (not usergroups, but 'social' groups). To be honest I'm not really very familiar with them, but I think a user can create a group that can only be joined by people who are invited.
ETA: You have to set "Allow Join-to-View Groups" under the Social Groups options to 'Yes', then when someone creates a group they can make the group "invite only" and choose to allow only group members to view content.
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