How to 'un'-remove someone from group?

I have a whatsapp public group and I removed someone. I can't work out how to un-remove them and let them back in because if i send them an invitation link they get a message saying 'you can't join this group because you were removed'. there isn't any way I can find to undo the removal action. I know I can 'add' them but is that the only way to get round this?