I don't think you can really measure the number of people using bitcoin accurately at any time because bitcoin is free to sign up and doesn't need any ID, and is mostly anonymous. You can measure the number of used addresses, but a person or organization can have multiple addresses.

Here: http://bitscan.com/bitnews/item/how-many-people-really-own-bitcoins-and-why-does-it-matter. It's quite outdated but you can safely say I think that there are around 2.5 million people right now using bitcoins. I think that's the absolutely minimum for now.