I'd like to get users from a certain Department. I've been looking for a while and haven't found whether it's possible or not ... I saw the http://spsite/_api/web/siteusers URL but it doesn't give us the Department.
You can query the user information list to get all the users for a site. Use:
http://siteUrl/_api/web/lists/getbytitle('User Information List')/items?$filter=Department eq 'Value'. This will return all the users with the department
Value for that particular site.