If a person has left a channel 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.
194
抱歉，如果有人离开了频道，只有双方互为联系人时才能把对方加回（双方都有对方的电话号码）。
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Lu-ho Lin,
May 3, 2022 at 05:50