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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抱歉，如果有人離開了群組，雙方必須是互為聯絡人（雙方都有對方的手機號碼）才可以把對方帶回來。
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Tony Wong,
Apr 26, 2020 at 16:05