Access Issues in WSS 3.0

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SharePoint

WSS

I have created a website in WSS 3.0 and also I have added(People & Groups Section) some groups(which are defined in Active Directory) to access the website.
Website authentication : windows
In web.config file :
Trust Level->FULL
Health Monitoring->False
The users those belongs to groups do not have access to the website.
But when I add those users individually to that website(People & Groups Section), they have full access to the site.

How can I give access to groups? What should I do?

Thanks for help.