It seems like I can't share a playlist/track to someone by their email address or spotify userid. Using Mac desktop client 0.9.14.13, I do "share", and then "send to...". If I type a spotify userid it says "sorry, no results could be found". If I type an email address it just disappears. I'm on versio. Similar on previous versions, on the Android client, and all other versions I've seen.
It seems like a very obvious use case, I see friends stumble over it quite often. On the FAQ it even says you should be able to share to a spotify userid under "send to".