SharePoint does integrate active directory accounts, of course, but how about security groups? Have a few sites where I'm fairly confident access is going through an existing Active Directory (AD) security groups (i.e. only an AD security group has been granted permissions through the 'People and Groups') In another situation, where I created the AD group and granted it permissions to a site, the customers were not able to access immediately. Eventually had to fast-track it and add the individuals to the People and Groups to keep the project going, but hoping not to have to maintain it that way.
Any specific requirements of the security group in AD? Universal, Global, or domain local? Is there any time delay between modifying group members in AD and having that take effect in SharePoint?