The GitHub API documentation says that the url
https://api.github.com/users
will give all users in the order they signed up, but I only seem to get the first 135
Both of the existing answers are 100% correct, but I would advise you to use a wrapper for whatever language you happen to be doing this in. There are plenty of them and there is an official one for ruby (Octokit). Here is a list of all of them.