Certain authenticated sharepoint users get "Access Denied" error

When we add our Active Directory groups into Sharepoint site groups, some of the group members receives Access Denied message. This is the problem faced in sharepoint 2007 and doesn't resolve even  individual members of the group are added. So, to get rid of it, just try through following steps:

• add users in DomainName/UserName format, instead of using just UserName. Avoid adding AD groups directly
• if you have hosted your site on port 80, and using different name apart from machine name as http://ABC, in this case don't forget to make a entry of your site in Alternate Access Path, under operations tab in central administration.

Hope, this post is helpful to you :)