It would appear to be Joseph Stiglitz (link is in French, or try this link in English), but do we know? Here is Grillo discussing the situation in English (translated). The link includes a dialogue between the two. Yet here (in Italian) is already some discord between the two, with Stiglitz accusing Grillo of lying by claiming that his party’s platform is based on Stiglitz’s ideas.
I cannot be sure which of these links to believe in its entirety, but at the very least there is an ongoing exchange between these two men.
I wish to thank numerous sources on Twitter.