Use the forum? Some people read the forum others create an account and participate in the threads.
Those that create an account get a unique identification number. Those numbers usually are assigned in sequential order, and if you find someone that created an account today you'll get a pretty good idea of the total number of accounts created on the forum. Now you have to account for how many single people have multiple accounts, and then adjust down that number of accounts by such an assumption to arrive at an estimate of the number of actual people that have created accounts.
Here's the profile link of a Newbie that registered on August 30, 2017:
https://bitcointalk.org/index.php?action=profile;u=1130390 At the end of the url of this profile link you'll find this unique identification number, "u=#######". In this case that number is 1130390 or 1,130,390.
My guess is there have been 1.13 million accounts created on this forum.
Now, let's assume that represents 1 person per account (we know it doesn't, but that's ok for now).
How many people own and/or use bitcoin?
The total number of bitcoin wallets is 16.6 million (
https://blockchain.info/charts/my-wallet-n-users?timespan=all). Now we all know that everyone (nearly everyone) has more than one wallet address.
Now, I don't have a good hunch on how to adjust this number down. BUT if we assume that each wallet represents one person than 10% of bitcoin users have accounts on this forum. It's a starting point...and less than I first thought when I answered the poll.