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.
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😔, if a 👤 ⬅ a channel, only a mutual contact can bring them 🔙 (they need to have 👉 📱 number, and 👉 need theirs).
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Deleted Account,
Nov 12, 2019 at 00:32