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 It's quite outdated but you can safely say I think that there are around 2.5 million people right now using bitcoins.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.
It is hard to say how much people are using Bitcoin because most of bitcoin users have more wallets and you can measure all the wallet from the world, byt we know that millions of people are using bitcoin.
To count how many people are there in a city is extremely difficult not to mention calculating how many bitcoin users are there all around the world so its gonna be a hard task for those who want to know about this. Therefore, why should we bother about the number, just need to know that its a very large number and its increasing everyday, every minute.