Is there any wiki/web service like Google sites which allows the admin to givepage level access/ authentication to the users ?
TWiki Access Control allows you restrict access to single topics and entire webs, by individual user and by user Groups. Access control, combined with TWikiUserAuthentication, lets you easily create and manage an extremely flexible, fine-grained privilege system.
PBWorks appears to offer something like this:
It's easy to share your workspaces with people inside and outside your organization. Quickly add users via email. You can also add people outside your organization as "guest users" who are limited to accessing just the content you specify. For large organizations, specify a trusted email domain, and allow anyone with a valid email address to join.