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，若用户退出了频道，只有双向联系人才能将他们加回来（即需双方互相持有对方的手机号码）。
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Taylor Fung,
Jan 27, 2020 at 06:11