Best I can find (from WaPo, Ironically) is that he was originally brought on by Bennet into the Carson campaign. After that, he was somehow added to a list by Sam Clovis and from there hired into the Trump campaign, supposedly without consulting Bennet (who said he would not recommend PapaD).
No one seems to be saying who brought him on - but my guess would be the campaign manager.
He's being a bit of a famefag - and while some amount of that is human nature, we all want to be cool and get some candy tossed our way - the direction PapaD is going now suggests that we've hit the poison in that well.