Getting all users in Moodle using the webservices

I'm developing a Java client to interact with Moodle. I want to get all the users registered within a Moodle installation. I found the webservice function called `core_user_get_users` which returns a list with users. When I call the service without any parameters I get a repsonse with a `invalid parameter` exception. When I add a criteria parameter (`criteria[0][key]=id&criteria[0][value]=some_id`) it returns a single user with that specific `id`. I can't seem to find what criteria to pass to the function to get the entire list with users registered.