English translation: notice to attend an extraordinary general meeting
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Explanation: The fact that it says "supplémentaire" indicates that this is an additional meeting to the scheduled annual general meeting (AGM), and so I think you would need to call it an extraordinary general meeting (commonly abbreviated to EGM).
I think in a letter to property owners, you would title it 'request to attend ...' rather than 'convocation', as I would think there is no legal obligation to attend, unless there is something specific in the deeds which legally requires owners to do so.