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
Please use since
parameter in your GET request.
https://api.github.com/users?since=XXX
Probably it's done this way to limit the resources needed to handle such request. Without such limit it's just asking for DoS attack.
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.
If you check the response headers for that request Github provides pagination links under the header Links
Link: <https://api.github.com/users?since=135>; rel="next", <https://api.github.com/users{?since}>; rel="first"
I believe since their api v3
Github has been moving towards a hypermedia api.
Github Hypermedia API
EDIT
This is beyond the scope of this question but its related. To learn more about hypermedia API and REST. Take a look at these slides by Steve Klabnik
http://steveklabnik.github.com/hypermedia-presentation/#1
You can filter on type:user like this:
https://api.github.com/search/users?q=type:user
See Also: GitHub API get total number of users/organizations