Legend is that Satoshi worked on the puzzle alone.
Then later, Satoshi contacted others for input that contributed to the whitepaper (Wai and others).
After the whitepaper was released, one other individual came along to help Satoshi build the software (Sirius).
After the foundation was established around mid 2010, many others appeared to help contribute (Gavin and others).

Bitcoin Core is the most recent version of the software, starting around 2013.