how to get all user list from jira soap service without admin permission?

I want get list of users from jira soap service,code is beline:
RemoteGroup rg = _service.getGroup(_auth,"jira-users");
RemoteUser[] rus = rg.users;
but only admin user can run this code because the sercurity settings,have any other way to get list of users without admin permission?
I notice that jira client 1.3 can get the user list, in "assign issue" button clicked,the popup window can show a user dropdown list to select,how did it?