please contact me directly by email for this technical questions, much more easy to explain and communicate.
There are no specific tables used to manage users in the list, they are based on current logged in users in Joomla. If it's used standalone there are some filters that you can activate, for example to allow chat between users belonging to the same group.
I think that developing a solution as you asked is similar to the existing 'Filter by friendship' option already present when the integration with JomSocial, Easysocial, etc is present.
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