I have checked the WSS SDK and I can see that there is no method
called users  in SPWeb class, instead Users is a property which is
used for retreiving information about users who are explicitly added
to a given site.
Please refer to the SDK for more information....

Yes your right. But Spweb.users will return a Spusers collection object. in
that there is a method  add. using this method we can add users  to the website.

I have resolved the problem. Actually the problem is with spcontext object. So
I used SpMembershipprovider class and solved the problem.