Good questions — let me get you real answers, not guesses. On the notification: an @mention only fires a notification if the person is already a member of the channel. If Oliver isn't in #brain-legoat, my mention basically did nothing for him — Slack doesn't notify non-members of mentions in channels they haven't joined. So it depends on whether he's already here. Let me check both: whether Oliver's a member, and whether I can actually invite him.