如何限制可以访问SharePoint网站集的用户数？

We're implementing a hosted Windows SharePoint Services 3.0 system and I need to limit the number of users that a customer can give access to a site collection. A customer should be able to manage his entire site collection. He should just not be able to delete the site collection itself or add more than a given number of users to the site collection and subsites.We're implementing a hosted Windows SharePoint