It is, after all, much harder to convince a group of mutually-suspicious humans than to convince one lone person.
That sounds right. Would you have evidence to back up the intuition? (This knowledge would also be useful for marketing and other present life persuasion purposes.)
#( TL;DR: Mo' people - mo' problems?
I can think of effects that could theoretically make it easier to convince a group:
(You could take preemptive measures against these worries, but Boxy might find security holes in every 'firewall' you come up with - an arms race we could win?)
#)
My comment was mostly inspired by (known effective) real-world examples. Note that relieving anyone who shows signs of being persuaded is a de-emphasized but vital part of this policy, as is carefully vetting people before trusting them.
Actually implementing a "N people at a time" rule can be done using locks, guards and/or cryptography (note that many such algorithms are provably secure against an adversary with unlimited computing power, "information theoretic security").
Update 2013-09-05.
I have since played two more AI box experiments after this one, winning both.
Update 2013-12-30:
I have lost two more AI box experiments, and won two more. Current Record is 3 Wins, 3 Losses.