I don't think that tagging is a good approach for this. Tags like these work best with real buddies who went somewhere together and then one of them took a pic and now he's tagging his friend on a social network., for example. And this can be irritating, too, if your friend cares about his privacy. I would ask my friend to share some of my posts and ask them to suggest these posts to their friend and like this it could go on. I think this is a better approach.