Clifford Riley is the author of many of the The 39 Clues extra series, such as Rapid Fire and The Cahill Files. He is supposedly on the Message Board, but none of his posts are allowed through moderation as he keeps posting his birthday. Rules are rules, as Admin Gavin B says.
His branch is currently unknown but given hints dropped in the Cahill Files, he is most likely a Madrigal (like Jude Watson).
Clifford Riley is actually a pseudonym for all the editors of Scholastic. The person acknowledged in each book supposedly written by him is the ghostwriter.