If a person has left athe group in the past, you need to be in their Telegram contacts to add them back.
They can still join via an invite link as long as they are not on the Removed Users list.
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Si una persona salió de un grupo, sólo un contacto mutuo puede volver a añadirla (necesita tener tu número y vos el suyo).
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Agustín Volpe,
May 2, 2020 at 23:33