Given the history of infiltrating activism groups, and the fact that Anonymous requires effictively zero effort to infiltrate, I'd put the probability at just a

Given the history of infiltrating activism groups, and the fact that Anonymous requires effictively zero effort to infiltrate, I'd put the probability at just a shade beneath 100%. That said, I'd guess that much of the irresponsible vigilantism of Anon probably is still the work of idealistic teenagers and so forth.