If a person has left the 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 group's invite link as long as they are not on the Removed Users list.
194
抱歉，此用户已决定退出此群组，因此您无法邀请此用户回来。
28/194
Deleted Account,
Jan 26, 2019 at 12:21
抱歉，此用户已决定退出此群组，因此您无法邀请此该用户已决定退出此群组，因此您无法邀请该用户回来。
28/194
Deleted Account,
Dec 18, 2018 at 05:12