When and how did Bitcoin start?
Bitcoin's design paper was published on cryptography@metzdowd.com on October 31, 2008, which is considered by most to be the official beginning of the project. On January 3, 2009 at 18:15:05 GMT the Genesis block was established. Six days later, Bitcoin 0.1 was released and announced on the Cryptography Mailing List.

It is hard to estimate the size of the community at any point, especially historically, but there is a decent timeline of the major events in Bitcoin history at the official wiki.