SharePoint 2013 - If Server Object model API used to get user form Site collection it throw error access denied even if Console application run context of Site collection Administrator

Hi

I create application which used Server Object model  API(c#)  to get all users , & create user in SharePoint 2013 site collection . If i run this application in Web application Administrator context it working fine but if same application run in  context of site collection Administrator it's throw error Access Denied .

This application logic works for SharePoint 2010 . So i wondering why application fail to create user in SharePoint 2013 even if i run this in Site collection Administrator context .

Is any setting is required in SharePoint 2013 to work Server object model API application or special permission require to site collection administrator to run this application.