There is no standard way to limit the number of users that can be added to a site. Not without serious hacking or removal of functions from users.
It is not possible to fix the problem, only manage it:
- Use the peoplepicker-onlysearchwithinsitecollection property with Stsadm.exe to prevent the people collector from returning only users who are already added to the site collection.
- When a hoster grants permission to a user to access a site collection, the hoster must add the user to this site collection.
Site owners can add other users to the site using their full names or SAM account names.
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