For users within an ArcGIS environment to be able to use Content Store, their ArcGIS administrator must first setup Content Store from ArcGIS MarketPlace.
Navigate to the Content Store item directly here.
From the landing page select 'Get it now.'
Fill out the necessary contact details and click 'Submit'. As Content Store is a free application to use, no payment information is necessary at this point.
The application will now be added as a piece of content into your ArcGIS Online environment, for you to be able to provision to specific users next.