user_list does not currently support pagination. It would be great if it did, so if you are able to contribute a patch that would be great. This should give a good overview of what's needed, and happy to provide furhter guidance.
Having said that, I'm not sure about the value of the list of current users in datahub.io. Even though we made a huge cleanup during the last migration, there are still thousands of spam accounts from when there weren't captchas in place. I'd say that something like 90% of accounts might not be legit.
You can check that browsing the UI for the user list at https://datahub.io/user (which btw you could consider scrapping if you really really want to get the list of users and not wait on the
user_list patch, but please be considerate)