I think it is quite probable that Bitcoin was written by a single person. It is not that complicated: block chain of merkle trees + signing transactions with public/private crypto + proof-of-work with hashing function + peer-to-peer networking. The first public release (0.1.0) was only 20k lines of code. Someone like Szabo, Wei Dai, or Hal Finney, who have each spent more than a decade thinking about and attempting to design decentralized currencies certainly have the expertise to design Bitcoin all by themselves.

My admittedly vague feeling is there are so many balances in the system that it was likely reviewed by a larger group, having lengthy expert-level discussions in a non-public forum, before release.

I've heard the theory that it's a government think-tank project that in asome sense 'escaped', in that it was thrown out to see what happened and has thrived beyond expectations.