I would assume that - by default - all members of the team at that weekend have an equal ownership. And not just of the code produced - of the concept as a whole.
Ideas and concepts are not usually considered intellectual property, and cannot receive protection or be assigned ownership.
Barring any agreement otherwise, the code's copyright belongs to the individual developer who wrote it, though in the case of software projects this may get a little weird when multiple developers write and overwrite each other's code.
The following is speculation, because I've read up on the finer points of copyright laws as applied to purely digital property, but by sharing the code the developer has obviously gave the person he's sharing the code with implicit permission to work and use that copy of that code, although, again barring any prior agreement otherwise, he should also be able to rescind that permission as he chooses.
There might be an implied right to use (but without a contract to specify compensation a court might require Billy (et alia) to provide some compensation), but if Billy was to re-license the code to a third party that would almost certainly be a problem.
