Sharepoint 2010 Hosting ... User Control shows all Users from AD

Hello,

I'm trying to set up a Hosted SharePoint 2010 Foundation-Environment.

When configuring permissions for a new site collection (xyz.sp-domain.com) there is the possibility to add users to SharePoint-Groups. But this User-Control shows every AD-user. But I don't want that customer A can also see every user name from all the other customers. Is there any possiblity to configure a root OU for every SharePoint-site-collection like it could be done in Exchange Hosted Organizations?

Thanks in advance!