"Danny has been a pleasure to work with over the last five seasons," said head coach, Steve Meehan.
"He has always given his all, whether it is during a game, in training or in other activities away from the pitch.
"Those are qualities that every player should have and I have no doubt that he will go on to be as successful in his new role at the club."
Grewcock has made 73 European appearances in his career, 39 of those coming in the Heineken Cup.
He made his International debut in 1997 against Argentina and has toured twice with the British and Irish Lions.