I've tried to create a rule where an email gets sent when a contact is "added" to a smart group. By "added" I mean the condition that smart group is based upon is met.
As far as I can tell it doesn't. It seems like it would be hard to do.
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Contacts don't really get 'added' to a smart group - it's more that when the smart group is used, a search is run to find who currently meets those criteria (glossing over some complex rebuild and caching details...) So there is no specific event for CiviRules to react to in a post trigger.
You might be able to base something on the 'Daily trigger for group members' cron trigger and maybe modify it for a more frequent run. See https://docs.civicrm.org/civirules/en/latest/trigger/